First let me say that I like to fly mostly JR stuff, it's just what I've always done and it works for me.
I've been using 4131's on the cyclic and collective, whatever on the throttle and the GY401/9254 combo and V110mm blades on the tail. I get a of minimum three all out hard 3D full tanks on one charge with a 1700mah AA NiCad pack. This combonation of servos has been working great in my gas machines since I've been flying them.
The 4131's produce 90oz./in. at .23/60 degrees at 4.8V, don't suck up a lot of power like digitals do, and are one of JR's most reliable and proven servos. The 4721 runs 119oz./in.
Some people like the extra torque of the 4721 on the collective. I had one and went back to the 4131. The 4721 got stripped gears from a bout with the turf gods. I decided that it was a pain to carry too many sets of different spare servo gears around. The 4131 doesn't seem to have any trouble horsing the collective around anyway.
IMHO, the GY401/9254 has excellent holding power for any machine. I ran a 601 for a while, but I couldn't justify the price for the performance gain. I do tail drops and very fast inverted backwards hurricanes and haven't had the tail let go since the 9254 hit the market. The GY601 did feel more solid and crisp, but for the price, I'll live with the softer feel and similar performance. I think you would really have to be pushing hard to justify that extra performance gain.
Now go out there and burn some GASOLINE!!
Life's a journey, NOT a destination.