We live here and have APs to both. We use both APs equally. As the others have said, both are completely different. Apples to oranges.
1. How does universal immerse you in a themed enviorment compares to Disney?
Disney is like being in another world. Universal feels like a theme park. That being said, I love both equally.
2. How does citywalk stack up compared to downtown Disney?
Citywalk is geared more towards drinking aged folks. DTD is more family friendly. But again, I still like both. As a family with young kiddos, I probably prefer DTD. But you have to walk through Citywalk to get to their parks and I always enjoy that stroll. Sometimes we stop and do a little shopping.
3. How will it be touring with out fastpass (we're probably not getting express pass)
In April, it's still going to be busy. It can be frustrating watching the express pass people walk by. But since you can't use them for Forbidden Journey or Rip Ride Rocket anyway, the passes are not worth it. It's a nice perk for staying on site though. You may want to get there early for those 2 rides. Otherwise, most rides are manageable. Spiderman may be especially busy right now since they just upgraded it. But either way, it's better than Disney. You don't really need a touring plan for this place.
And overall how does a Uni experience stack up against Disney?
Again, I love both equally. Disney is more magical. Universal/IOA is more exciting. Disney is more about planning, Universal/IOA is more go with the flow.
I would highly recommend eating at Mythos at IOA. The prices are good for a sit down and it's REALLY yummy! Make an ADR just in case. And plan to eat an earlier dinner. It closes early.