If you plan on using this bike to do anything other than ride it to a friends house dont get the viper, I don't even think I would recomend it for that... No I have never riden one so dont ask, but looking at the specs alone you can tell this bike aint no better than probably your average walmart bike. For starters, there is only 3 pieces of chromo on the entire bike, the seat tube, the axles of the pedals, and the one piece crank. You should be looking to a bike as close to full chromo as you can, you probably are not going to find a full chromo bike for your price range, but I'd look for a little more than this. Secondly, the wheels only have 3/8" axles, that may be acceptable for the front (if it was a decent wheel) but for the rear you want a 14mm axle. A one piece crank is complete garbage, and the 44/16 freewheel set up is prehistoric, and cheap. On top of all this the bearings are all unsealed, do yourself the favor and dont buy the viper x, its litterally a rolling turd.
As for the redline, it is a far better bike, but it is a race bike. If you dont plan on racing I would advise against this one also. You are a bigger rider (not talking crap) and I dont know If I would trust a cheap aluminum frame. That frame probably wont hold up to much abuse, and keep in mind that aluminum dont bend, it snaps. If you plan on racing this bike is definatly the one to pick of the two. If you plan on freestyle with these bikes, I'd recomend steering clear of both of these, but that being said you are in a price bracket where it is going to be difficult to find a quality bike period, if it HAS to be one of these two bikes, you should probably go with the redline. This is assuming your riding dirt, if your riding street I'd save some more money before buying anything.