No airbrush is "easy" to control! Hand to eye co-ordination, air pressure, viscosity of paint and unsteady hands all conspire against the poor user! Practice, practice, practice is ultimately more important than what style of brush you purchase. While a better tool often results in a better job, the hands holding the tool dictate how good a job it really is. The results of investing money in a high-end AB pays off over the long time, not the short time. In other words, a $50 hammer can bend a nail just as fast as a $5 hammer! The arguments of single-action vs. double action, siphon vs. gravity is many and varied and akin to Ford vs. Chevy!
My preference is double action as I feel that the potential (as of yet not fully realized by yours truly!) is greater than single action. Those who really know how to use a single action airbrush can put the lie to that, very quickly.
Typically gravity fed brushes allow for thinner lines simply because you can use less air pressure, so the argument would be that gravity fed allows for more control. However, some people don't like the cup on top of the brush, as it interferes with their sight line, potentially reducing control. You can buy a AB like the Badger 100SG which has a teeny cup built into the body, but it holds such a small amount of paint as to be useless for HO scale. The Badger 100G has a small cup on top of the handle, but only holds 1/16 oz, again, probably not suitable for HO scale! Another alternative would be the Badger 100 SF, which holds slightly more paint in a cup that is held off to one side (LH or RH models available) giving you the sight line of a siphon fed, with the control of a gravity fed. It also has the benefit of a rotating color cup which allows you to spray vertically or horizontally.
Confused yet? I know I am!
I highly recommend that (if possible) visit a store that sells airbrushes and hold each in your hand and imagine using them. If there isn't a store near you, perhaps an detail auto body shop, or sign shop might be a source of hands on experience. Then go to
http://www.dixieart.com/ and
http://www.bearair.com/ two highly regarded and inexpensive online stores to buy the model you want.
For the record I have used Badger models as examples as that is the brand I am most familiar with. AFIK they are the only manufacturer that offers a side feed airbrush. Paasche and Iwata are also highly regarded by many of the forum members. Myself? I own a Badger 150 siphon fed, and have a 100LG on hold with a fellow member who is selling off his excess ABs. That way I end up with both kinds, the siphon fed that I can attach a HUGE bottle to for those long sessions, a gravity fed that will allow me to do detail work, and the parts are interchangeable between them!