The Mustang is decidedly a less "polished" car than the Toyota... I personally can't stand the '94-04 Mustangs. I think the interior is cheaper than the one in the Fox-bodies, even with all their age-related squeaks and rattles. One of my friends' mothers had a brand-new '97 Mustang, and never once did I think "That's a really nice car" when riding around in it.
However, the 'Stang does have lower miles than the Celica, and if it has been well-cared for and garaged, would be the better buy. There were lots more Ponies on the street than Celicas. Make sure the top and seals do not leak, and have a mechanic go over it.
I think everyone should own a convertible at some point in their lives. You will either fall in love with them, or you will never buy one again. But it's just amazing to toss back the top and go cruising on a warm summer day, or out to a drive-in on a crisp fall evening for hot fudge sundaes.