One big subwoofer or two weaker subwoofers?

Size is not the issue. It is the quality of the subwoofer.
Two (stereo) subwoofers is generally better; however two mediocre subwoofers is not better than one excellent one.

Since all REL's perform well, two Strata will work very very well. However, if you think someday you'd like to have two Stadium, then get one now and save up for the second one..

Second part is how good are your main speakers?? If two (or even one) Stadium outclasses your main speakers, then don't waste your money; get the two Strata..

ask your rel dealer ,or better yet.. rel north american distributor......rel stentor 3 ,will smoke 2 stratas,or 2stadiums it will even beat studio ii(old model)...just no contest .You read alot of threads, yes 2 should be better than 1 ...it is usually for same size or similar make.BUT when you say bigger (hope better)like stentor 3,this thing is fast, deep ,DEEP,moves air big time ...you can't compare even 4 small subs..Besides stereo doesn't exsist for deep bass . YES i do have one,SEAMLESS

Sugarbrie's first paragraph makes an important about quality. Cheap subwoofers are worse than none, as a rule, since they tend to degrade the sound from the main speakers.

When it comes to subwoofer design, however, one large powerful subwoofer isn't necessarily the best approach. Subwoofers with several smaller drivers (in the 8-10" range) have the virtue of being able to move a lot of air, while the smaller drivers have less mass and thus can respond more quickly than a large driver in the 15-18" range. Multiple drivers in a subwoofer (assuming it is well designed) also offer the advantage of averaging out the resonance points, thereby yielding a smoother frequency response (Vandersteen subwoofers, for example, use three 8" drivers).

The primary advantage of using a stereo pair of subs is that they load the room more evenly, thereby reducing the resonant nodes in the room. A stereo pair of subs also presents a much more realistic sonic image, since deep frequencies may be coming more from one speaker than the other. (When I bought my first Vandersteen subwoofer, I was very pleased with it, and bought the second one expecting it to just add a bit more "oomph" to the deep bass. What I experienced was a MUCH better overall sound reproduction that provided spatial clues.)

Assuming your budget allows you to buy a good quality of stereo subs, I would personally recommend that approach to having a somewhat more expensive single sub.

There are so many issues to consider here. What is the cross-over point? What are the room modes like (2 subs make cancellation of problematic modes much easier)? Is it 2 channel or multi-channel? Here is a link to a paper we wrote on loudspeaker placement. Loudspeaker Placement on Audioholics It may answer a few of your questions--may raise a few more.

When I trying to decide which way to go, I called Sumiko, the US distributor for REL. They stated without a doubt, that one Stadium III was better than two Storm or Strata III's. I purchased a Stadium III and couldn't be happier. I'd go with a sub that has flexible cross over points. REL's go down to 22hz. I never thought I'd have my sub set as low as 23hz, but that's were it ended up. Like a previous post stated, a lot does depend on your main speakers.