Hmm hard decision.
Long tables are relaxed and look lovely, however people's conversations are limited to fewer people. Agree with Lawwa, round tables allow for people to be a bit more social and talk to more guests.
We had long tables of 10-14 guests and an advantage of them is that you can lengthen/shorten them at the last minute, which is what we did as there were a few people who did not show at the last minute, and the decorations and centerpieces still looked good, we didn't notice the tables were all different lengths. Whereas with round tables it might be more challenging to accommodate varying numbers of people. Hope that made sense, not sure if it did