It's hard to do both, so I have to say it's going to more likely either-or-case.

I tried doing a side-project during PhD, but quickly stopped because it was eating up my progress in PhD.

I guess, for me, my desire to finish PhD was greater than trying to do a startup.

What about you? How much more do you want to do PhD than startup (and vice versa)?

I have to say, completing PhD is.. an extremely rewarding and worthwhile experience, albeit... a tremendously stressful thing to do. :)