I only went to last year's
MVMCP and I have attended this and last year's Grinchmas. Here are my comparisons:
Mickey's Very Merry Christmas Party
Pros
-Hard Ticket Event, which means fewer guests in park during day = shorter lines
-Special Fireworks
-Snow
-Free Photo, Hot Choc + Cookies
-Lots of characters
-Xmas Parade + shows
Cons
-Hard Ticket Event, which means you have to pay to get in
-
NO AP DISCOUNT
-Some features of the party will be for all to see in the MK closer to Xmas
-Getting more crowded every year
Grinchmas
Pros
-Included in admission to IOA
-Kids get in free w/ Publix promo
-Special shows
-You can see the Grinch obviously
-Snow...and you can play with it too
Cons
-This is the only part of IOA that seems to be in the Xmas sprit
-Age limit for Mt Crumpit and Snow play
-Since SL is painted brightly, it is kinda warm (on warmer days)
-Grinchmas is CROWDED on the weekends
-Other than Grinch, other characters are not as accessible