Everyone seems to have a different view. My old mate Ernie Lee for example, sticks to 72nd scale kits of any era, his aim being to produce a model of every aircraft type - even though he knows he'll never live long enough to do it! Then there's Neil Robinson who used to be a 72nd scale man, but he shifted to 48th scale when his eyesight started failing! He's still terminally obsessed with Spitfires though!
I used to stick with 72nd scale modern RAF/Nato subjects but then as I got older (and the aircraft started to disappear) I realised that I was primarily interested in the stuff I'd grown-up with in the 1970's. But then the huge increase in availability of 48th scale stuff happened, and it was too hard to resist. Then the same happened with 32nd scale... Eventually I realised that life was too short to obsess about a particular scale or type of aircraft, as it seemed pointless when you considered just how many aircraft there are, compared with how many you actually build models of. I decided I'd just go for anything that caught my eye in future!
So these days I just buy a kit if it's an aircraft I like, or if it's just a good-quality kit. I'm still geared towards my favourite types (particularly things from the 70's era) but when things come along like the new Tamiya Spitfire, the kit is so good that I can't ignore it just because it's a Spitfire! I think the only stuff I'd never really bother to buy would be WWII Luftwaffe stuff. I've never seen the fascination with them at all. Or even more obscure things like WWII Japanese aircraft, or really unusual biplanes... things like that would bore me silly!
I'm just waiting for a top-class injection-moulded 32nd scale kit of the Varsity... might be a very, very long wait...
