Semi-random thoughts:
First, join a Triumph club if there's one anywhere near you, such as
Vintage Triumphs of Wisconsin (web site seems a bit out-of-date, but....) and talk to Spitfire and GT6 owners; maybe even see if they'll let you drive a car. See if either car is really what you want; especially, make sure you're comfortable after 5-10 minutes!
I'd be hesitant to just jump on either of the first two cars you see, unless one is the perfect condition, deal, color, reminds you of your childhood, softly and seductively beckons you, etc.! Even then, that usually is not quite enough to cause you to rush into anything.
As noted, the last of the Spitfires is somewhat strangled by emissions equipment and added weight due to mandated safety equipment. But they're still loads of fun for what they are: a relatively inexpensive and basic convertible sports car. Acceleration and top speed aren't everything, of course! As nice as the late (1973) GT6 is, it also is somewhat strangled compared to earlier ones, especially 1971 and before. (I know: I had first a 1970 and then a 1972 back-to-back. Both were totally stock, and I loved both, but there was no ignoring the difference between the 79hp '72 and the 95hp '70!)
And yes, there's the "Spitfire 6" or "GT6 convertible" or however you wish to look at it. But this is not something for someone new to Triumphs. There are more than a few of these about; all are "homemade" cars, as it's not something the factory ever conceived, built or sold. They can be great, but more than a few have been slapped together out of junk parts. Ideally, such a car should have ALL the running gear of the GT6, including suspension and brakes, but many don't. You need someone who really knows both cars to help you determine if such a car is built properly.
Bottom line: take your time. Don't rush into anything, lest you find yourself on the debit end of a major mistake. :frown: