I have reloader 15 and 19 and 90 grain speer hot-cors. Alliant powder web site has loads for both powders with this bullet. Has anyone tried both? Which one would be better?

Try both. Your rifle will tell you.

the 243 will prolly shoot both of em reasonably well the only powder I shoot thru my 243s is either of the 4350s but Ive come to the conclusion that 243s are very user friendly with various powders.

I run r15 w/75gr bullets and r17 w/90's. If I were you I'd try both 15 and 19.

+1 on the 4350 powder! I use 41.5 (IMR) under a 95gr NBT. Awsome deer load.

I use H4350 in my 243 and have gotten spectacular results with 100gr bullets

My preference of the two you mentioned would be RL 15 with a 90 grain bullet...