I try to TRX. I found that I have same problems with you (weak core muscle). I can do machine upto 15 -20 rep. However, It is difficult for me to 10 rep for TRX. TRX is very difficult. I want to challenge it. It can train my core muscle. I plan to add TRX in my trainning
I go to Cali and normally will spend 20 mins running, 30 mins weight training on upper body (machine), and 15 mins work on abs (sit-up, abs crunch, etc)