Depends among other things on the terminal where you arrive and whether there is a HEX stop at the terminal, whether the next train you get is the express or the regular train to London, and how quick you are when changing in Paddington.

If you check the times on nationalrail.co.uk and tfl.gov.uk, it's essentially the same (50-60 min from T1-3 and 55-65 min from T4 or T5).

I tried both in the past, and would now go for the Piccadilly line: even if it is 5 min slower, the fact that you don't have to change is worth the 5 min.