Audio Research Ref 3 VS Mark Levinson 38s

I've owned both of those preamps, though not at the same time. I had the 38S for about a month, around 15 years ago. Then I spent about 3 years with the ARC Ref 3, and sold it about 8 years ago.

In my system, to my ears, there was no contest at all. I much preferred the ARC Ref 3, more resolution, more musical, more natural.

Again, that is just my opinion, and I have been known to prefer a tube preamp. Very few SS preamps have sounded natural to me, with Klyne the possible exception.

The 38S stood out to me as a very HiFi sounding unit. It checked many boxes, good bass, good highs, but it never sounded relaxed or natural to me. Could have been system synergy, or whatnot, I can't really recall what else was in my rig back 15 years ago, but I did turn around and resold it rather quickly, and the guy who bought it really liked it, so whatever. That's why they make so many flavors.

That's just my $0.02.

A piece of advice DO NOT base your purchase on someone else’s opinion. Let your ears be the judge because it’s you that will have to live with it.

+1. As the OP can see, there are those here who feel the ARC is a much better choice, and there are those who feel the ML is a much better choice.

In the end, you are paying your own money, you need to choose what sounds best to you, not what sounds best to someone else.

In my post above, I actually had 4 "high end" brand preamps in my home at the same time for a couple months so that I could choose which one I liked best. Along with the ML 38S I had a Krell KRC-HR, BAT VK-51SE, and a Conrad Johnson Premier LS-16 mk II.
In my system, to my tastes, they finished in this order:

1) BAT
2) CJ
3) Krell
4) ML

I am sure that order could be made up in many different orders.
The point is, choose what sounds good to you, not what sounds good to someone else.

Would the ML 326s be worth the extra 2 grand or so ?

Only you can make that decision.
I am sure that you will be happy with either one though.

I would think that if you are paying an extra 2 grand, the price is either really good on the ARC Ref 3, or really bad on the 326S.
I see them selling for about the same price, unless the 326S has the optional phono stage, and even then it's only about $600-$1,000 more.

Now that is judging by sold prices on HiFiShark, not asking prices in current ads.
So if you are seeing a 2 grand difference, either the asking price for the 326S is too high, or the asking price for the Ref 3 is too low.