I'm a big fan of the .309 JDJ. I run a 165 Ballistic Tip @ 2400 fps in my 14" SSK barrel. It's essentially the equivalent of a .308 Win. in performance, but can be utilized in the Contender, and the .308 cannot. I've used mine to my complete satisfaction for about 15 years or more. Of course, I've never been anything but completely satisfied with any SSK product.
Dies are only available from SSK. Loading is easy after necking the .444 case down once in a .308 Win. die. I've used RE15 for years, but JD has always recommended the 4350's in this cartridge.
I once had a T/C 12" .30-30 Hunter (i.e. ported) barrel rechambered to .309 JDJ, but sold it after a short while. Neither the velocity nor the accuracy were satisfactory to me in that particular barrel.
It's a shame the .30 Merrill (.30 / .225 Win.) is not available in the Contender (to my knowledge, but MGM might be talked into it). I believe it would be as superb in a Contender as it is in a 14" Merrill or XL, and the brass would be stronger than .30-30 AI.
Failing that, the .30-30 AI is a good choice, and a lower recoil, slightly less powerful alternative to the .309 JDJ.