i dont think all the S2s are more powerful.
I know the xynos chip is more powerful but the skyrocket has the same cpu as the vivid which is a qualcomm unit.   so he can buy a really fast cpu phone and not have LTE which is a tradeoff (i'm on verizon and well i wouldn't make that tradeoff if it came down to say a hypothetical evdo phone with an xynos vs LTE with a qualcomm s3).  I think honestly part of the reason I didnt like owning a galaxy S was that it was just too blatant that samsung had 0 originality copying apple.  I mean I don't buy all the patents BS about say making a square phone, but touchwiz they clearly were aiming for the look of a particular other device.
that said,  an S2 cpu is definitely faster than a 1.2ghz snapdragon s3 if he did get a non lte SGS2 but there are other factors.  the S2s all have 800x480 screens if you don't want pentile matrix.  and they have unnatural looking colors.  sure its a nice looking screen, but you can't get qhd (i dont think they even make a qhd phone) or 720p screens on a non pentile samsung phone as far as i know.  and the feel / build of they feel cheap in hand.  you know that thing you hold phones with.
i mean thats still my personal opinion.   the whole build quality thing, and i know a lot of geeks don't care (i've spent hours arguing about say an audi interior vs say one in an equivalent priced mustang which has way more HP... so its the same damn argument)
		
		
	 
s2>Vivid
skyrocket=vivid in terms of power
HSPA+ is more than fast enough, id rather not have it and save on battery
Design comes down to preference, and i actually do like the way HTC designs their phones (titan looks awesome), but im at the point where i just want less design, small bezel, and huge screen.  ICS can put the buttons right on the screen, and im sure ill buy a case for whatever phone i use, so i really just want a huge beautiful screen, with nothing else.
like someone mentioned, qhd is a dealbreaker.  720p phone owners will undoubtedly feel some growing pains as apps change over to the new res, so id rather have a really nice standard res screen, or at least what the new standard is going to me.  
As far as samsungs screens go, i would say 99 out of 100 people would choose thier screens over the competition.  Even my sgs2 screen is absolutely beautiful at a lower res.  I cant wait to see the nexus' screen.  Colors are vibrant, and blacks are deep.  Do you really think your going to notice pentile on a 720p screen that the nexus has?
I personally could care less how it feels in the hand... because if your using your 500+ dollar device without a case your asking for trouble.  My phone feels fantastic with the case i have on it.
As far as the software on the phone goes, the less the better.  Sense and TW annoy me, so im currently running CM7.  If i wasnt, id at least choose a different launcher.  
IMO, HTC's offering on ATT are crap at the moment, so dont blow your upgrade on a crappy phone.
	
		
	
	
		
		
			With all of this said, if OP really wants  an AT&T HTC phone my advice is to buy a Inspire off contract (like  Craigslist or ebay or something) and wait till next year to use the  subsidy when AT&T has better HTC phones to offer. Something with  ICS, 720p, Quad Core, etc.
		
		
	 
This was my dilemma.  I hate having an upgrade and not using it right away (im stuck on att, so the longer i take to use the upgrade, the more money im giving them)
I caved and the the s2, and if dont end up liking it, ill sell it and get the nexus.  
Usually i buy whatever iphone is the newest and sell it.  So i look at an upgrade as a 400 dollar credit for me to buy a phone.
This is why i hate when people waste an upgrade on a phone that really isnt worth much. Example:
Someone going and buying a htc inspire for 50 on contract, when they could have bought it for 150-200 on craigslist.  (att giving you 100-150 dollars)
I got my s2 for 50bucks, so if i sell it ill probably get at least 400 (+350)