I can share our reasons for T4 v T5...
Firstly it was budget, when we were looking, a late T4 (in good condition, low mileage etc) was in the region of 20-25k €, the cheapest T5s circa 2005 were 25k+. So initially we didn't even consider T5, but along the way we did see some of these early T5 models and quite honestly they were falling apart (we only looked at 3 examples and there was always something broken/missing... One was in such a sorry state we came to the conclusion it could only have been stolen/recovered - you couldn't imagine anyone would mistreat their own vehicle like that!)
Secondly (having seen those examples of T5) we considered the T4s to be more robust - ok maybe not as 'stylish' (and of course beauty is in the eye of the beholder!), but they're bulletproof (don't take that literally!) - also we figured a late T4 (that's been through model changes/improvements) will stand the test of time better than a first generation T5...
Thirdly, we simply preferred the look and feel of the T4, just a little more 'retro' than a T5, more reliable than anything older from the VW family - in all honesty I'd love an old T3 or even T2, but I don't have the time to spend or worry about will it breakdown on the next trip. The T4, if maintained well and looked after, should run easily 400-500k (there are many examples of this), the 2.5tdi is a workhorse and probably one of the best engines (before all the electronics were introduced), it'll easily cruise at 110-120kmph and the fuel economy is not that bad...
Other than our very brief experience of T5s I can't comment further on them...
So i guess it comes down to personal taste and budget primarily. I'm not knocking the T5, we just felt for our budget we got more of what we were looking for in the the T4...