use the (stock) captivate and do the "average" tasks of checking email, texting, browsing the web, searching the market, checking out stuff via the facebook app, etc.
then do all of that on the HTC inspire. it will be better, and it will be faster.
only thing you can really argue for the captivate is the better screen, but even that is considered to be subjective by many.
The screen is better on the captivate, but in all honesty, the inspire screen is a beauty and the average person will have a hard time telling the difference unless they are holding both phones in their hands at the same time and even then some people might actually like the Inspire better.
As far as the phone itself. Well, I use my phone mostly for Email via my company's exchange server. It takes 5-10 seconds after clicking the email to actually open it.
There is a general lag to the phone, opening aps, going to screen to screen, and sometimes when typing. A reboot fixes most of these issues for a while, but within a couple hours, back to lag.
Battery life - 6 hours max, and that is just limited use (email, a couple texts, facebook and twitter updates, 3-5 5 min phone calls, etc.). Mostly background stuff. No wifi, no surfing the net, no actually updates to twitter or facebook from the phone, no videos, ect. Nothing real crazy. Having to charge my phone in the middle of a work day is just unacceptable. Got this phone in July, got a new battery in August, and it it just getting worse.
I even went and flashed the phone with a couple differnent ROMS that claimed better battery life and while they did increase my batter a bit, I still have to charge around 6 or 7 hours.
NOTE: When I say have to charge I mean I am usually 15% or less.
The Inspire is fast. Everything zips along and I have yet to see any lag. HTC sense blow TW away. Of course, there are replacements, but...
The phone seems more solid to me (build wise).
Bigger screen does make a difference.
Battery - Over the last 2 days at work, I come home about 9+ hour days and the Inspire battery is holding strong at 61% and 55% on those two days. Basically same amount of use as my Captivate with some variation of course, but I never had my Captivate hold up anywhere near 60% after a work day. Best I ever got out of my Captivate was 30% at the end of the day and that was basically no use of the phone at all.
I loved my Captivate when I got it. I told everyone who asked me about phones, go get the Captivate. It was my first Android phone but I was really impressed, and in some areas I still am, but the battery and lag are just unacceptable to me.
Go try a Inspire...if think it cant get any better with the Captivate, you are going to be blown away by the Inspire.