BIG difference, although both go quite well with test. The deca is going to help any sore joints/aches/pains, will put on some good size and strength but I also find I get a fair amount of bloat from it.
Fina on the other hand tends to harden me up like crazy with veins everywhere. Strength goes through the roof but size increase is minimal to moderate.
Application wise, deca you can hit as little as once a week where fina needs EOD or ED injections.
"They" say it's not good to run the 2 together as they both tend to convert to progesterine thereby increasing your chances for those type side effects.
KR