Tim, you will find that everybody gives a different view!
I'm very lucky having one of each. I started of with a Phase 2 in Liquid Yellow with a straight through exhaust, induction kit, big brakes and Treadline seats, and found it to be incredibly involving to drive, unlike most 'modern' cars. I liked it so much that I had to buy a Phase 1, which came along in Mars Red. This one is completely standard apart from a modified gear lever linkage, and is equally as enjoyable and involving as the Phase 2.
The Phase 1 is perhaps a little more focused than the Phase 2, with throttle response and clutch being more immediate. I found it quite difficult to drive smoothly in traffic as first, but moving the throttle pedal away from the brake pedal by 2 cms (now the same as the Phase 2) made it a bit easier with my size 12 feet!
The performance and handling is no different in the real world, driving on the road. Maybe on a track you'd find the difference, but both cars feel very planted driving quickly around twisty open roads at speed. The Phase 1 has been reported in the press as being dangerous and unforgiving right at the edge. So is the Phase 2, and every other mid-engined car, but you'd have to be driving like an idiot, or not used to rear wheel drive / mid engine, to get into trouble.
I can't see any difference in build quality between the two models. Both of my cars are low miles with original paint and have been well looked after. There is certainly no rust anywhere on either car, on top or underneath. The Phase 1 cars tended to be used more as everyday cars when they came out, so many had 5 plus years driving in the winter, and have some corrosion on the brake, suspension and subframe components. A lot more Phase 2 cars were bought as 2nd cars it seems, so there are more low-miles and cossetted ones around than there are Phase 1 cars.
I haven't heard of either car having to have bodyshells welded due to rust yet. The interiors in both of my cars are like new, and given that one is 14 years old, the other 11, that's quite impressive.
I think that if I was buy one today, it would be a Phase 1. A car in excellent condition is about £8k to £10k cheaper than a Phase 2 in the same condition, but really good cars don't become available very often. That price difference really isn't justified at all, and I can see in years to come that a really top Phase 1, which remember is much more rare that the same condition Phase 2, will command a higher price than the Phase 2. There is also more TWR content in the Phase 1, having been designed, developed and built by TWR.....that may make a difference to Brit fans of the car! There is also no doubting that the bug-eyed look of the phase 1 is pretty cute, and of course, in the collector car world, the first of most models is usually the one that appeals the most.
The most important thing to do is to join the club so you have access to all the information, and if you find a car you want to buy, ask about it on the forum first. Many cars are well know, and some have had a hard life, even though they may look all shinny on top.