an App that meets the Ops needs is not a Synth App, in fact an iPad App that does meet their needs is not one that I have found.
We know very little about the OP's needs. Only that it is for live performance, has to be user friendly, and he needs good strings and brass. (And we didn't even know
that much when you said that an iPad made no financial sense and that he should get a Kross or Juno DS instead.) Which of those requirements do you think cannot be met by an iPad?
The fact is I doubt that the Op will find what they are searching for without buying a keyboard and a Tablet
Okay, so you've changed from saying he should get a Kross or DS instead of an iPad (because the iPad approach doesn't make "financial sense"), to now saying he should get a keyboard AND a tablet, which is great, but of course, is more expensive than EITHER of the other solutions!
The fact is I doubt that the Op will find what they are searching for without...forgetting the MIDI controller and iPad route for now or by changing their requirements.
What requirement has he put forth that you think need to be changed?
Cons is you need to buy many apps for different kind of sounds. I am not sure if you can find sweet brass ,string sounds. I am obssessed with them.
There are single apps that have lots of sounds, you don't necessarily need to buy many apps. But to get "best of each" for a variety of sounds, you may get more satisfactory results from multiple apps. If you're going to be running multiple apps at once, it's preferable to have more RAM. All reasonably modern iPads have at least 1 GB RAM, but to run multiple apps, I'd suggest a model with at least 2 GB RAM.
Questions:
* Besides strings and brass, which other sounds are especially important to you?
* Do you play one sound at a time, or do you need splits and layers?
* Does it matter to you whether you would call up your iPad sounds from the keyboard's front panel controls vs. calling them up from the screen of the iPad itself?