What is better 1 bigger sub or 2 small ones

i have Dynaudio C-1's. I have been told by one dealer to go with one better sub. The other dealers says to go with 2 smaller subs. I am only Interested in Rel subs. I am going to call Rel in Berkeley tomorrow and see what they say.
I'm afraid to say that there are some tradeoffs that should be considered. Many will claim that multiple subs are always better than a single one, since it will provide more even bass throughout the room. While the bass uniformity will be better with multiple subs, the speed and depth of bass reproduction is also dependent upon the quality level of the sub(s) being used. So there are cases where a single higher quality sub will outperform multiple lower cost subs in a given room. I don't believe that there is a hard and fast rule about this that will apply to all room setups.
(1) This is entirely room environment dependent. Without actually test-driving the alternatives personally, you are only guessing .

(2) Multiple subs are harder to set up properly and

(3) multiple subs are usually introduced in a large room for HT ..... Not for 2 channel.
As with most things...it depends.

Good larger subs generally go lower with flatter frequency response. A single sub will sound more different in different parts of the room in many cases, so there is more of a sweet spot. Two subs enable more balanced response in more locations within the room concurrently.

Also the right sub depends on what speakers used with. To fill in the low end effectively with speakers that are more full range already, you will need at least one large sub.

So to get the best of everything in theory you want two larger subs. Good ones two.

If cost is a factor, then you have to determine what compromises you are OK with and decide accordingly.
Its complicated, as all of life is. I have two, which I started with. Felt one would never tell me if two were not better. I am happy.
multiple subs are usually introduced in a large room for HT ..... Not for 2 channel

Ridiculous statement.
Agree with the, "it depends", viewpoints. However, do not forget that subwoofers are speakers too. So do not percieve or treat them with any less consideration than your main speakers.
Multiple subs are harder to set up properly

I also disagree with this statement. I agree that it is possibly more difficult. But not necessarily. Once again, it depends. For example, if the room is such that placement of a single sub disallows it to ever be properly set up, that is, no matter where you place it, you have noticeable issues (i.e., frequency suck-outs and/or boominess), then a second or multiple subs might be used to eliminate those issues which a single sub cannot. In this case it is then easier with multiple subs to "properly" set up.
According to the book, "Get Better Sound", 2 are required for accurate imaging, and all the old saws about how bass is non-directional are beside the point.

Theoretically that is true, but for the lowest audible frequencies that a sub would deliver in an already mostly full range system, how much it matters practically is debatable I think.

That using more subs enables smoother bass response throughout the room is a significant advantage nonetheless, if that matters. 4 is even better than 2 and 2 is better than one and the results in most cases should be clearly audible. Audiokinesis is the vendor in these parts who designs some of his speakers in a modular manner that allow for this by design up front.
I use two REL subs, which are positioned close to each speaker. It's important not to 'overuse', and spend time working on intergrating them with the speakers.
Finding a good dealer to help you put them in will reap huge benefits. I used Tim at 'Tone of Music' in San Francisco, and love the transformation that adding them provided.
Can 2 overload some preamps?

good to hear you selected REL subwoofer(s). These are the best in the market! The most musical, easiest to integrate in any set-up. Start w/ 1 sub for awhile. Get used to it and allow it attempt to transform your experience.

1 sub is certainly the easy set-up. 2 subs can present a more challenging aspect. Your room is going to play the major role here- is it "live" or not.
Keep me posted & happy listening!
I use 2 Rel T7s with my Cremona Ms with great results, highly recommend a dual sub set up.
Will you be using the sub(s) as bass extension from 2 channel audio, or as a .1 dedicated sub channel?
The frequency response of the main speakers needs to be the first consideration for sub matching, whether for one or two. My Maggies start rolling off around 50 Hz, so my small, fast subs are a good match even though they only extend down to around 36 Hz.

However, the C-1s measure down into the 30s, so the sub(s) should be strong from around 35 Hz on down into the 20s to turn your rig into true full range.

We don't know your budget; you could possibly get a pair of JL E10s for around $3K and get meaningful bass down into the mid-20s, or you could get a $3-4K single JL that would take you down to 20 Hz flat.

The main thing is, for the C-1s, don't get subs so small they don't add meaningful bass extension and strength to what the C-1s can already do. From there, get two if your budget allows, or one if it doesn't.

I position my subs symmetrically alongside my Magneplanar 1.7s so there are no phase or imaging issues. You can also get good imaging if you center a single sub between the speakers so the mains and sub are all equidistant to the listening area.
Look at the specs of the different REL models and get the one with the lowest
possible frequency rating you can afford. My trusty old single REL Q150e
(10" mosfet little monster) is located directly behind one of my main
speakers, is all the sub my room can take, and it sounds great. 25 to 50hz is
where it lives (I've tested it with a tone CD...I recommend trying that as the
results are interesting) and the theory of "non-directional" low bass
is actual fact in all but the largest and deadest rooms...however, 2 subs will
make sure neither of them works very hard, it will put a smile on the salesman's
face, and will make you appear to be a very serious hifi owner to others. I
suggest you try one REL and after dialing it in simply see if it's doing its job to
your tastes. Instead of buying another one, spend the bucks on that REL wireless
gizmo....a seemingly great idea.