If you are first timers, spring for the two day, two park. The Universal parks are every bit as big the Disney parks, and there is a lot to see in one day. You may sell yourself short trying to see both in a single day.
Depends on how old DS is. My kids started transitioning to Universal from Disney at about 12yo. Once they had mastered the tame rides at Disney, they were ready to move up to the big rides at Uni. Now that they are late teen/young adults it's Uni first, maybe Disney. If you are a coaster junkie and enjoy getting wet, then Universal is the park for you. Also, their simulators (especially Gringotts and FJ) put anything Disney has to shame.
Best tip: Unofficial Guide to Universal Florida. They will give you the lay of the land and enough information so you can handle the park like a pro your first time out.
So, Love it:
- more thrill rides
- better theming (HP worlds esp).
- faster touring (smaller crowds)
- better CS food
-better park hopping (you can walk, or take the Hogwart's express between the two parks)
Leave it:
- Not as magical as Disney
- For most of the TM's, it's a job, not a lifestyle (they are still nice, but real people, when they smile you know they mean it).
- not as good for small children (Suess is good)
- for some reason about 50% of the rides will get you wet (not recommended for a cool cloudy March day)
- no FP+; you can get FOTL for extra $$
It's a wash:
- Parking $$ hurts at both
I'm sure I'll think of many more reasons once I hit post.