If it’s just for studio use, I would avoid all Yamaha Arrangers as they have very poor Midi Controller functions. (Great as modules though)

Unless there are some specific features that you require and can only get with Arranger Keyboards, (Easy play Easy Setup etc.) then I would get a quality Midi Controller keyboard, Computer, Arranger software, (There are many out there) Good Sequencer and some VSTs, as you will find the quality is way beyond anything that can be achieved with any hardware arranger. (You will need to put the work in though)

Another alternative is to use a Workstation(Kronos, and Motif etc.) instead of a Midi Controller, which will also have a quality way beyond any hardware arranger, and while still more involved will probably be a little easier to setup than full software setup.

If you still want the Yamaha arranger route, than having compared all 3 side by side I would avoid the T3 as while the SA2 voices were great, the rest of it was blown out of the water by the previous T2, the T4 on the other hand is a different animal altogether, and if you like the way Yamaha do things, and sounds they produce, then it is one of the best arranger keyboards out there. Values wise the Yamaha 900 series take some beating, (Which is why a lot of players take them over Tyros) as they only have a little less than the Tyros models, and still retain that popular Yamaha sound.

Korgs arranger keyboards are interesting as while they are not as easy to setup as Yamaha (You will need to put a bit of work in to get them sounding as you want) they are way more flexible, and go way beyond anything you can achieve on a Yamaha Arranger, however they still fall way short of a Workstation or software based system, but are certainly worth having a look at.

Hope the above you helps you to make a decision, but in the end only you can decide which will suit you best. (Just make sure you look at all options)

Bill