I bought a second tent in the middle of last year, and did a fair bit of research prior to doing so (much of it by reading past WH threads on the subject, and asking for and getting advice in the "gear" section of WH). It's well worth your following up on all the tent threads - there's a goldmine of experience-based advice there from folk who do a huge amount of camping.
For what it's worth, here's a distillation of the conclusions I formed.
1. Unless weight is an absolute major consideration, it's worth getting a tent one size up (in terms of person capacity) than you believe you need - ie a 3 person tent, if you expect to need to accommodate up to 2 people, a 2 person tent if you expect to have to accommodate one. There is very little increase in weight if you go one size up, but a massive increase in convenience/space. Many very experienced and regular hiker/campers commenting on this site go for the larger size, accepting the slight increase in weight.
I have a Vango Banshee 300, which I got about 5 years ago, and a Force 10 Helium 200. I use the former for camping near the car, or if there are 2 of us; and the latter for solo wild camping. I have slept 3 in the Banshee a couple of times - me and my 2 grandsons (ages 8 and 11) sardine-wise - and it was tight. Adults would be seriously intimate!!!
2. There's a big advantage in having a pitch-in-one tent (ie the inner is attached to the fly sheet) so that if it's raining you don't get the inside of the tent wet. I guess if you are very slick it may not make a big difference, but I've been so thankful more than once that the Banshee goes up as a oner.
3. It's worth having a bathtub groundsheet. It's not always possible to find an ideal pitch: I've more than once had no option but to take a pitch on soggy ground, where I'm quite sure that I'd have had water coming in over the edge of the groundsheet had I not had a bathtub design.
4. Check on your minimum acceptable clear internal height. Not being able to sit up straight can be a bit of a pain. A number of folk have made this point in tent threads, and having paid inadequate attention to it in opting for the Helium, I'll be more careful if there's ever a further purchase.
5. If you anticipate having to cook in inclement weather, make sure there is a sufficient porch area to do so. People have commented that some tents do not have sufficient space.
6. Most groundsheets seem to be watertight, whatever their rating. Mine are rated 6000mm and 5000mm respectively, and, as far as I can see, can "float" on a marshy pitch without letting in water. It's often damp the night after, but this seems to be due more to condensation due to warmer air from the sleeping bag.
7. You don't need to pay large sums for a perfectly serviceable 3 season tent (I'm in agreement with Rockhopper on this). I paid about £120 for my Banshee a few years ago, and I have no complaints at all (indeed I've camped in reasonably heavy snow, and it's shed the snow quite effectively). Having said that, I wouldn't vouch for it in a howling gale - I don't think it's designed for this - but I aim to avoid such conditions, not least because it's damned difficult to sleep when it's blowing hard! I'm a tightfisted Yorkshireman, but it makes sense to me to be very clear exactly what you're paying for over and above what you get in one of the standard Vango tents if you decide to pay £300+ for a tent. You can get a lot of good gear for the £200 - £300 you save
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8. Be clear whether you expect to hike with the tent, and factor weight into your evaluation accordingly. I was surprised at the variation in weight for similar person-capacity tents.
Good luck!
