krutiparekh:
In general, later rounds tend to be tougher as more seats are filled and admissions rates tend to go down. R3 (June) isn't quite as difficult as R4 (July) but obviously not as easy as R1/R2.
I am not sure the correlation between years of experience and rounds is the right one. It's not as if the earlier rounds are skewed towards younger or older candidates...or least now that we're aware of.
Pros: you can still apply and go later that year and it's not the LAST round; and if you're more ready to apply in R3 and can put forward a better application than applying earlier, you'll do much better in the process.
Cons: a supposedly more difficult round than R1/R2; potential visa issues depending on your personal situation; scholarship money isn't as available as in earlier rounds; harder to claim you're bringing a unique profile if you don't actually come from such a background given the applications in the previous rounds, etc.
In general, we always recommend people apply to the earlier rounds if possible but want them to be entirely in the right place and time if they have to wait for a later round.
I'm sure you'll get a slew of other great thoughts to complement ours. Hope that helps!