My family and I stayed in BLT with a lake view this past August - it is the absolute best resort we have stayed at so far!! At first, we were upset because there wasn't a MK view available when I booked. However, after staying there, we had enough oportunities to see the MK from the hotel that we didn't miss not seeing it from our room. To watch the Fireworks, we watched from the floor where the elevators are - they have places to sit there and they weren't crowded - just me and my family. We also watched the Fireworks from the walk way bridge from the BLT to the Contemporary - that was a little more crowded, but was able to really enjoy the FW's and get some good pics from there.
I would suggest the Lake view and the use the money you save for something "magical" while you are there with your family.
Enjoy which ever one you go for!
Charlie