Well, I thought I'd update in case anyone's interested.

I ordered the Focusrite from Musiciansfriend: $499 PLUS I'll be getting a $100 coupon back towards my next order. They have a pretty generous return policy, so if it doesn't suit my needs or sound good, I have 45 days to send it back.

How did you like your ISA One? In the years since this thread, what have you come to learn about various preamps?
 
I was looking at these two preamps as well and I went for the cheaper ISA. I think it's oversized and ugly but it does well with condenser mics, though not much better than my other preamps. It has been good for low output dynamic mics when I do louder vocals but exhibits more hiss than I'd like when recording normal to low vocals (spoken word to breathy singing). One characteristic I like about the ISA One over my cheaper preamps is that while they have similar noise levels, the distortion on them is bad at higher gain levels. The ISA One has been practically distortion free. Have these been your experiences as well?

The ISA One is a good preamp at a good price.  It was my first stand-alone "pro" preamp.  For $400 it is hard to beat, esp. w/ the extra features you may or may not need.  You shouldn't have any hiss - or I haven't noticed any, even when running a cheap ribbon.  While you might not need any of the extra features a really cool one is to use the transfomer on the mic preamp and still have the DI available.  Perfect for recording a singer songwriter.
 
It is easy to dial in the right amount of gain w/ the variable and switchable knobs.  And it certainly doesn't crap out easy like most built-in pres when you get someone going from 1-11, either on the amp or in their pipes.
 
As far as sound, it is a great middle of the road goldilocks w/ a good transfomer.  Not too colored, not squeaky clean.  Not as highly spec'ed component-wise as the $1000 preamp crowd, but I haven't heard that hurting it much.  You are paying double (or more) for a red-hair more sound.  That said, I use my RND Portico II first on just about everything, but that has more to do w/ the extra features on the channel strip, not the preamp itself (tho the Neve sounds "bigger," tho most of that is just expectation).  The only lower priced unit I like better is the Warm Preamp, which is thicker (just like the ISA is transfomer thicker than IC preamps) and more "vintage" sounding.  The Warm Tone Beast is more expensive, but has a bunch of different sounds available as far as gainstaging, op-amp and transfomer choice.  Not as clean as a grace etc., but you can add all the hair you want on top of that clean sound.
 
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Another ISA owner here.  I've previously owned two ISA Ones, and currently own an ISA Two.  I've never owned a Grace (although I've been tempted many, many times), so I can't really compare them.  Gotta agree with everything AT posted above regarding the ISAs - it's a lot of preamp for not much money.  If you're the hardcore gearslutz-type who demands that incremental increase in performance/sound/color, then be prepared to spend a little-to-a-lot more.  But, if you want a solid, reliable, clean and very flexible preamp for a great price, there is little that can compete with the ISAs IMO.  The ISA One has the advantage of operating as two pres - e.g., one for vocals and another simultaneously for guitar.  I didn't really use it in that capacity, but it sure is a nice feature to have, particularly for home/hobby studios.

I am on the fence for getting the ISA One. I already have Peavey VMP2 that has plenty of warmth - great pre. Looking to add something a little cleaner but still retains some warmness to use as an alternative. 

I bought an ISA One a couple of years ago and use it on  nearly every project now.
 
It has enough gain available to suit my cheap ribbon mics and to my ears, it's completely distortion free and retains natural dynamics & warmth pretty well

I think once you start using the Focusrite, you'll forget about the other Pre's unless there's a "character" or sterility you're looking for that you're not getting for some reason. Learn it's features, and benefits to their maximum potential, and enjoy it to the fullest! I think the Focusrite will balance the two for you. Good choice.
 
And, yes, there are endless arguments over which pre is best. You have to take such a thing with some serious chunks of salt, even from the magazine reviewers. But it's actually a good thing to see so much competition. In fact, the more competition that's out there, the more fierce the arguments over the most minute little differences, many of which don't stack up to anything a consumer/music lover will fight over. 
 
As many others will agree the money should go something like this (for hardware): Mic first, pre-second, A/D third, cabling 4th (although cable-routing is paramount).... Power conditioning is very important, but mainly for tracking and just to eliminate noise in the audio tracks. So, these last elements may still be debated openly. That's perfectly ok.
 
I have several great pre's, and love them all: API-512c, Chandler Germ500, GAP Pre-73, Summit 2BA-221, True Precision 8, Audient ASP008. Each one of them are different, but great at the same time. What I'd concentrate on here is how they are built, designed for what, and then I will listen to them all, and shoot them out with different mics. A tedious, drawn out process, but well worth it if you have time--and fun!
 
But without auditioning the Focusrite, for the circuitry it employs, there's lots of care and taste put into it to make it a real mic-pre. I've used one and loved it.
 
Oh, and price means relatively nothing to a pre's character or true quality. For example, someone might pay tens of thousands for a mic pre that was built in 1970 by a specific human that just happened to be called in for engineering a project by a band who's music garnered them a global popularity, Grammys, and fan-fueled staying power for 30+ years. The buyer may be unaware the quality of that mic-pre may have worsened slightly over the years, rendering it inferior to a brand new pre that emulates it.
 
Happy times recording!!