There's the old, iconic hardware and the newer NES 2. The NES 2 is highly sought after because it's reliable and the older models can be finicky, but as others have said, if you replace the 72 pin connector they will likely work perfectly. I don't have much experience replacing them myself but from what I understand it's not very hard -- plus, any retro shop with its salt will be able to do it for you for a few bucks. The new connectors are usually 8 to 10 bucks at the most. Much less than the cost of an NES 2 on ebay. Plus, the NA version of the NES 2 has ONLY the RF out, whereas the original has two-cord composite out for a better vid signal (no games are in stereo anyway so two-channel sound only matters if you just want something coming out of that other speaker, in which case a $4 splitter will do the trick).
Sooo, long story short the model 1 is the way to go for most people. The extra cost plus inferior connections are big negatives. (Plus I think they look garish but that's personal preference.
The main thing to do when buying a model 1 is test it and make sure you don't get a ton of black screens and blinking power lights when trying to turn it on. If that's the case the connection is likely shot. Also try to test the tray that holds the games and make sure it has a good amount of spring to it and isn't worn out because that can lead to poor connections, too.
A good trick for getting a finicky game to play is to pop it up, and gently wiggle it back toward you slightly, enough so that it just barely clears the plastic edge below when you push it down. Blowing in the cart will only do so much good - you're better off cleaning with some rubbing alcohol and q-tip. If you need to blow dust out do it at an angle so you're not just blowing the dust -- and your spit -- down into the cart.
One other thing I would mention is the accessories. IMO the only one you need is the Zapper. The Advantage doesn't have many advantages because it's not clicky. The Power Glove and ROB, while a neat bit of trivia, only worked with one or two games. Pretty much every other controller is worthless. Some people like the Max, which had a knob you could move around, but it was kind of a gimmick because underneath the knob was a regular old digital input.
Might have more to add later, but this is getting long for now ...............
