Each has its own merits.
In the summer, we wear less clothes and I feel more out of sorts about being overweight and the motivation to lose is strong. It's so hot, though, that I just don't feel like working out and getting overheated. I don't like to cook when it's hot and my appetite isn't as great.
In the winter, we can hide under more clothes but the baggy sweaters can only do so much to hide weight and can also make me look bulky and huge. It's much cooler in winter, so exercising is more fun, but I also feel like cooking and eating more often.