I just got the Wii Active, have had the wii fit since it first came out and now have the plus.
I like the Active because it's a good workout (Was sore for 3 days after I did it), it really feels like I'm exercising. The Wii fit I like because it's fun.
I use both in my weekly exercise routine.
My mom, who's close to 50 (don't tell I told), loves her wii fit. She's suppose to be trying out the active but I haven't convinced her, she did tell me something where she's pretty much just exercising is what she's currently looking for.