Both features are wonderful tools and once you learn to use them you'll find that 10 to 20 seconds between songs seems like eternity.
My best advice is to go to http://www.psrtutorial.com/lessons/playing/regist/10regGD.html to discover all these keyboards have to offer with their registrations.
Good Luck,
Gary
Ditto to Gary's comments.
Maur welcome to the forum.
Here's my 2 cents, first I'm sure Yamaha Corporation has the tools to build a music finder that will accomplish quite a bit more, including adding registrations. All they have to do is look at Korg and it's Songbook feature. That feature basically acts like a set of registration files, but instead is stored in a database similar to the music finder on Yamaha arrangers. Songbook stores everything from key changes, OTS changes and just about any other parameter you'd care to adjust. I went from Yamaha to Korg a few years back loved Songbook, but it just wasn't enough to keep me from coming back to the Yamaha brand.
I use a combination of registrations and the music finder database. I don't use the stock preloaded MFD Yamaha supplies, instead I create my own custom MFD files. That works like a charm because on s910 and Tyros4 you can load a style from any location right at the keyboard. Simply press "add to mfd" and an on board, 3rd party or midi file can be loaded to the mfd ( I believe mp3 files can too, but I don't use that option) . As far as I know that feature was never available on older Yamaha arrangers.
As far as registrations go I most often use them to store banks of instruments named, piano, organ, strings, guitar, brass, trumpet etc. the list goes on and on it's entirely up to you how you set them up. The sky is the limit once you get the hang of it. Sometimes getting the hang of it is frustrating. I know I've been there. Good luck.