We did the party last year, and DS ( 10) has asked to do it again. Then I saw a deal for Legoland and he was intrigued, and with the cab ride there, the price would be comparable. We only want one big added expense-so-WWYD?
I would say that 10 is about the max age for LL (although my almost 12yo DS would still go in a heartbeat), so if you're going to do it ever, do it now. We enjoyed MNSSHP, but given the choice I'm pretty sure DS (and DD9) would choose LL.