Do what feels right for YOU!

the ONLY downfall to a surprise wedding at your 21st is that some people may decide it's just a birthday, it's for a younger person, they won't come (esp the older generation), whereas they would come to a wedding. so you're left with people missing that might have otherwise been there for your big day... but ultimately it's about you and DP, so you need to decide whether you're happy to have some of those people missing kwim?