for me, both have their dangers! winter is probably tougher because i go and watch sport each weekend - which throws out eating patterns (and offers many tempting high fat/cal foods) and where i am likely to drink. i also find myself hungrier in the cold weather.
but summer is when i put all my weight back on - i used christmas and my holidays around christmas last year as an excuse to eat myself silly and the bad habits just continued on in the months after.
i'm more likely to exercise in winter though - i go really red when i exercise & in summer i feel really self-conscious (silly, i know) and i have always played a winter team sport so it's kind of habit.
all that typing & no real answer to the question
i'll just say it's as easy or as hard as i choose to make it, no matter what the season