Sorry, in reading back my last post I realized it came off more stern than I meant it to.

The fact is I wanted to give you some kind of help but I'm not very familiar with guitar effects.
For your keyboard, I think mostly what you want is a good reverb sound, and then after that maybe chorus for sounds like strings. However, the string sound is probably already "chorused' in some way so adding another chorus effect might make it worse. Some of the boxes have a rotary speaker effect, which would probably sound good if your keyboard has an organ sound. If it were me, I would base it on what made the best guitar sound and leave the keyboard as a secondary consideration. All the boxes you mentioned have reverb so it's really just a question of which has the best and smoothest reverb. And that might be a related to the price.
Here are some catalog prices (US) of the stuff you mentioned:
Zoom GFX707 $160
Boss ME-30 $230
Line 6 POD $300
Boss GT3 $400
Digitech has a few, also. And how about the DOD VGS50? There are so many. Like I said, I'm not familiar with the sound of any of these. You might get a better answer asking on the guitar forum. Many people here swear by the POD for guitar. I wonder if it really has a decent reverb sound, though, since they put so much of the emphasis on guitar distortion. Besides, you need to spend extra to get the POD pedalboard. But I suspect that in general, the more you spend, the better the sound will be for keyboards.
Don't get fooled by the "analog" terms they use. Without opening them up, I'm pretty certain all of these are digital. (Although, it looks like
the DOD VGS50 does have a tube preamp, so you can call that real analog) When they advertise an "analog distortion" sound I think they mean that they a simulating an analog distortion sound using a digital processor. (It's a program running on a digital signal processor, a DSP) The POD is digital and it's not crap.
You need to be an old fart to know what the word "patch" means. It comes from the old days of modular synthesizers. You would set up sounds on one by connecting (or patching) various modules together with cords (called patch cords). So once you set up a sound you liked, that sound was called a "patch". Since then, synthesizers have become more integrated so that you don't need to connect the different parts with patch cords, but the term has been carried over and we still call a sound a patch. But on something like
a PSR310 where the sounds are not editable (I don't think?) you don't call it a patch, just a sound.
Jim