If you are genuinely faster than your mates, and you want a bike that’s comfier and a bit more fun than your current hardtail, I’d get a decent travel full sus bike personally. Something with a decent amount of travel (think 5″ or more both ends), circa 30lb or so, big disc brakes, 2.3″+ tyres etc. This will slow you down a bit on the ups (and also make you fitter still!) for your mates to catch up, but it’ll be a whole heap of fun on the downs…
A Hardcore Hardtail is going to be more fun to ride than your current bike on the technical stuff and the DH, but it’s not going to be any more comfy. I’ve got 3 bikes right now, a lightish steel hardtail with 120mm forks, a lightish full sus XC bike, and a fairly burly long travel (170mm) full sus bike. Of the 3, if I had to do without one, as much as I like it, it would probably have to be the hardtail. Now I’ve had a lot of hardtails that I really like over the years, but ultimately I can do everything I can with a hardtail on my full sus bikes, only more comfortably (and hence for longer, and more enjoyably most of the time)… This is all in my opinion of course, others will disagree. Mountain biking is a hobby that is highly subjective and open to personal preferences though, as you probably well know.
Either which way, whatever you do, building it yourself is a good way to learn more about the mechanics of bikes, and how to fix things yourself. Which is the best way to keep more money in your pocket for spending on more bike bits in the future!