I prefer X-SAMPA because I understand it a little better, but ARPAsing has more English resources and Moresampler compatibility. ARPA also seems to sound a little more realistic while X-SAMPA is more mechanical.
There's also VCCV if you're okay with recording that amount of samples.
Honestly, whatever you're comfortable with. In terms of difficulty I'd say ARPAsing < DELTA < VCCV