Servos - Coreless or Digital?
Basically, the only factor is how much precision do you want; and are willing to pay for?
Digital servos aren't any stronger or more reliable than their coreless counterparts. They are, however, much more precise.
Some will say that the "average pilot" cannot tell the difference. Bull. I am certainly no more than average in my flying skills, and I could tell the difference big-time.
I have two models, an X-Cell 60 size helicopter and a Kangke CAP 232 Sport airplane (also 60 size), that I flew for years with coreless servos- JR 4131s in the helo, Futaba 9202s in the airplane.
When I got a 9Z, it came with 9252 digitals. I put them in the helicopter, and could not believe the increased crispness and smoothness; especially on collective (main rotor pitch). So I bought four more 9252s and put them in the CAP. Again, the model's "feel" improved dramatically.
The downside to digitals, of course, is their cost, and the fact that you need to use PCM to get the most out of them. They also use more energy than standard servos, so you need to plan your receiver battery capacity accordingly. I've been running a 1650mAh NiMH pack with four digitals and two analogs, and I get 5-6 flights per charge. My helicopter (four digitals, one analog) has a 2400mAh ni-cad, and it is good for four flights- probably more, but helicopter servos get more of a work-out that those in airplanes, and I'm very conservative with my $1500 machine...<G>
I think digitals would be wasted expense in a typical "sport" plane, like a Stik, etc. but if you have a model that can utilize their added precision, then they are really nice. Just don't ruin the experience with Z-bends or some other junk linkage hook-up. Invest in ball links; you'll be glad you did.
Steve