Both are institutions..
YCcombinator is in SF bout tech solutions.. Better valuations in 2nd rounds.
Techstars is in multiple niches in different cities.. NYC is about Media & Ad-Tech.
Really depends what your startup actually needs.
But seriously those 2 accelerators are like the diamond in accelerators..
You need to prepare separately for each of them, know mentors & startups teams from each, communicate for months to follow their requirements and keep upgrading few times till you apply multiple times or generate enough traction and call the mentors through Clarity.fm with needed confidence before applying :)
Accelerators do not care of ideas, eventual team built or some beta with small userbase nowadays, especially YC & TS. Generate revenues, exponential growth and network among their offline events so you have any chance to get in.
But make sure to follow your niche mentors & accelerators, do not choose just the very famous 3 :) Good Luck.