Hi,
as I mentioned, I'm only a hobbyist.
I've been a keyboard & style making addict for years.
My 4 most recent ones have been 9000pro, VA7, Technics KN7000, my current one is the SD1plus & I'm probably going to get either a Tyros 2 ( if husband doesn't faint at the price tag ) or a PSR3000. So to a certain degree I do my homework.

Main reason I personally wouldn't have bought the Midjay is, being an arranger, is one of it's functions, it's not necessarily it's major function. For an entertainer who uses midifiles, mp3's & arranger functions,
it would probably be perfect (I'm not aware of any other module that can do all of this stuff) it's a jack of all trades.

You mention Discover 5, automatically midifiles & midifile manuipulation springs to mind. (I don't know anything about it's vocalizer as I also can't sing).
Unless you're really good with midi , midiing stuff together, especially if you're not using a dedicated controller, could be a nightmare, not so much because of the midjay, but maybe the discover 5, you may want to try & find out if anyones used it as a controller..

I've midied my digital piano to my sd1, but my piano is straight forward, doesn't hve sequencers etc that may complicate a setup.

You may not be into editing or creating styles or using sampler etc, but you only have a certain amount of styles onboard. ( 128 I think??) Midjay can apparently load SD1 styles etc, but editing options are fairly limited.
Even the sd1 doesn't have any kind of event list editing to fix up a style, like I had on my other keyboards , but at least I have the option of event list editing a style in a pc sequencer & then recording that style across to the sd1 via midi.One big plus is , you can create your own styles via a simple mix n matching of style parts. ie take drums from one style & copy it into another etc etc. That's a simple way of adding to your style list. These are some of the functions that the midjay doesn't have as far as I'm aware.
I had considered using EMC styleworks for editing the sd styles to suit the midjay. I'm glad I wasn't relying on it 100% though, I've discovered it can do some pretty strange things. Great program, but not 100% ideal editor.

Can I make a suggestion, check some of the older posts on this forum.

I don't find the action heavy on the sd1, but I'm also used to the fully piano weighted action of my clavinova.

The SD also has lots of dedicated buttons. 3 intro's the 4 variations etc etc, check out the layout of the Midjay & make sure it would suit.

best wishes
Rikki