The GT6 and TR7/8 are entirely different cars, which may make your choice a bit easier. Before you decide, carefully determine exactly how you intend to use the (restored) car. Daily driver? Weekend day trips? Cross country exploration??? The answer to these questions will guide your choice.
I hate to admit it but the GT6 is not the most comfortable car on the planet. I think you really have to love a GT6 to use it as a daily driver, daily commuter or long-trip excursions. The GT6 is a true GT car that's best enjoyed on weekends and sunny days for short trips. The cockpit is small, hot and noisy. A/C is relatively unknown for these cars. And since the body sits on top of the frame, there are inevitable squeaks and rattles, especially on bumpy roads. However, some folks (like me!) actually love this "antique sports car" feel in a GT6 that you're literally part of the machine. A GT6 delivers this sensation wonderfully.
On the other hand, the TR7 is a more modern vehicle that's far more comfortable to drive. The TR7/8 is not comparable in any way to the GT6, IMHO. For example, I wouldn't hesitate to drive a well-restored TR7/8 across the continent. They're a pleasure to drive, it's easy to install A/C and other creature comforts. And the cockpit a tad small for bigger drivers, but quite comfortable. A hard top makes for better highway trips, too.
For these and other reasons, to have a GT6 and a TR7/8 is an excellent combo! /bcforum/images/%%GRAEMLIN_URL%%/thumbsup.gif