Always prefer OTB.
OTB is more focussed + distraction-free as well as honest (you pretty much know you are not playing an engine .... unless the stakes are really high :))
Interestingly enough, I always seem to remember OTB games from memory way better than the 2D/Online ones I played ... even though they were the same "slow game" time control where I pretty much used the same time mgmt in both formats. Has me thinking that the tactile "touching/feeling the pieces" + the whole "chess club atmosphere" conditions the brain better to absorb every essence of the game.
If you are fortunate to live near a chess club (which I am!) and can actually make the time (which sadly enough, I no longer can!), consider it a blessing as this is fun, social (you make a ton of friends) and feels way more enriching!
When I started playing chess, I ONLY played games online. no one around me played, and I didn't feel confident enough in my abilities to go to a local club to play otb.
After a year or so of this, I started participating at a club near my college, and found it extremely difficult to calculate and visualize with the real pieces, not a perfect overhead view, etc. For a while I actually really disliked playing otb, simply because it was more difficult for me.
Now, I almost refuse to play chess online, simply because I prefer otb so much more. The visualization issue eventually worked its way out, and I don't feel the pressure or necessity to win while playing online.
Which do you prefer? and why?