I'm generally one to advocate for the lower-priced models, but my experiences with Crosmans and Gamos have formed the exception to that rule. I suppose it does depend upon your intended purpose. For just a fun plinker, a Walther Terrus may be a good option. If you are wanting more power for hunting and are willing to shell out a bit more money, Umarex or Hatsan are likely your best bets. Given what I would guess is your price range, I would opt for a Umarex Octane or Fuel (both gas-piston breakbarrels, just different stocks). My only complaint with the Umarex's that I have handled is their triggers (better than Crosman and Gamo, but far from match grade), and the fact that both rifles are black synthetic stocks. That's just my personal bias, mind you- I like a solid, classically-styled wooden stock, but I must admit that synthetic is much lighter.
Another option in the lower-priced magnum category would be Benjamin's NP line (they are made by Crosman, so the triggers aren't great, but the rest of the build quality is better than their price range), and they have the added benefit (for me, at least) that they do offer wooden-stocked models. My biggest gripe with Benjamin would be the fact that almost all of their rifles do not have open sights; this is another personal preference issue, but one large enough that I refuse to buy one. Still fun to tool around with if offered, but I can't justify spending money on a rifle and being forced to spend more to put usable targeting devices on it.
I will admit to being a confirmed Hatsan fanboy, but I will also concede they are NOT entry-level rifles. Much better than their price, and the fact their velocities are tested with lead pellets is endearing, but the recoil and noise make it difficult to learn the basics. It's pretty hard to learn good shooting technique if you keep getting punched in the cheek. Plus, the fact they have a tendency to be a bit overpowered makes plinking less of a fun exercise and more of a danger. I learned that after my Hatsan 135 punched through a watermelon and then through my garage wall- 30 meters behind it.
As for the second part of your question, I will admit to having a preference for gas pistons over metal springs. Some people say metal springs are more powerful than gas pistons; some say the opposite. What is known is that gas pistons function better in bitterly cold weather (living in the Midwest, I can tell you that is a fact), and do not suffer if left cocked for long periods of time. As far as longevity, high-end spring guns will last a lifetime; so will mid-range gas pistons. In the lower end of the price range, it gets a little more sticky; the gas pistons can give you more velocity and a longer lifespan IF you do your research and get a good one. Cheap metal springs can fatigue quickly or break outright; very cheap gas pistons can leak. Overall, if I were shopping in the bargain category I would go with a good gas piston. On the whole, I would go up just a little and get a REALLY good gas piston.