I've not been at either times myself, but I would prefer to plump for late April seeing as the weather should be kinder then. Weekends always seem to be busy, but if you go mid-week, the queues should be minimal as the Easter holidays are earlier next year.
We went in March last year and the weather was great. We had to buy factor 50 for the kids. Probably just a one off though. Going again in March next year, so hoping the weather will be the same.
Easter weekend is 21 -24 March. the schools in some of Scotland are not off until 7 - 18 April. So watch out, could be quite busy.
We've been 3 times in March and it's always been cold, sometimes very cold, that's not to say that there hasn't been some nice weather during March but we nearly always manage to miss it.