Why not both? Apparently, Appo was the one to lead the 501st into the temple, indeed meaning that Rex dropped out along the way. Maybe he simply left the army when Ashoka did the Order, taking up a life inspired by Cut who settled as a farmer.
Funny, there has been so much speculation as to whether or not Rex and Ashoka, Bly and Aayla had something going. If they did, I imagine the clone walked away.
1. We saw Bly kill Aayla in Ep3 2. Because I showed Bly kill Aayla in detail in my first gig working for Lucasfilm, the "Reversal of Fortune" web comic that went along with the original CW micro-series and Ep3
I've done 7 web comics for Lucasfilm, the first 3 were connected to the CW micro series and Ep3. the 4th was the SW/Soul Caliber crossover comic and the final 3 were Seasons 1-3 of the Clone Wars. all were on starwars.com