The philosophy of backpacking is not, is this 3lb or 5lb tent better, but to eliminate consistently all weight from all items. Besides, the usual practice again is to split up carrying a tent, poles to one person and the tent to the other. If you're in shape, your pack is good and fits well, and snugs to your body, and your shoes are broken in and fit well (extremely important) the difference between 40 and 50 isn't really felt. However as you might notice at this point, that's quite a lot of ifs.
Frequently people forget how much water they're going to be carrying, and how incredibly heavy enough water for a long hike + cooking a meal is, despite the reminder from the Fremen that the best place for water is inside your body. Same goes for food, which I also don't recommend skimping on. What you really discover while backpacking is how little stuff you actually need to live, (silverware = pfff!) and when it comes down to having a clean change of clothes or staying dirty and having a snugger pack, it's not a difficult choice to make at all. With that in mind, never, ever skimp on socks or boots. (also make sure your tent is, in fact, waterproof
)
The rule I used when buying tents was like shakran mentioned, 4 person tent = tent for two people plus their gear.
Then again, I went backpacking for 14 days at 10,000 feet with a 60lb pack, so what do I know ;p (I was appointed group pack-burro)
oh! and most importantly, don't forget toilet paper.
good luck and have fun!