They see me snip'n they hate'n
In truth,
They all have advantages and disadvantages, but, the only thing they have in common is they are not cheap.
All can be made to be quiet, all of them can be made to be accurate. Gas is easier to adjust/harder to maintain, bolt is harder to pull back, aeg has more fail points unless overbuilt.
If you like doing it, any of the rifles will work for you.
The greatest tool you have as a "sniper" will be your radio, not your weapon.
If you must go cheap - the KJW M700 is a great starter if you like to tinker, or a $100 Bolt rifle that is marui-spec if not. Both can be upgraded into monsters over time.
Try and borrow a bolt rifle and try it out. many are willing to lend them as they are very robust. You will know within 5 minutes if you like Bolt/AEG/ or gas better haha.