This will show you improvement in loading things. Booting window, loading roms, etc. If this isn't a current concern at the moment, then it's money wasted.
If you're running modern PC games that have a loading time associated with them (SFIV, for example), then it'll pop up faster. But most games in MAME (any non-chd) and any other non-iso based emulator are loading the entire ROM into ram, and thus after the initial the speed of the disk is mostly not important (if you don't have enough ram and windows starts paging to disk (the swap file) then you can see things slow down while it's writing).
SSDs make great boot drives, and then traditional hard drives for all the big files. This is how I have my cabinet set up. Windows/Hyperspin/Videos/Snaps on an SSD, roms on two mechanical drives. Windows boots to Hyperspin in 14-15 seconds (including bios time).
I wouldn't worry about 3gbps vs 6gbps too much. You're not loading data of a sufficient size for this to make a noticeable difference. I'm not even sure that many of the currently available drives even take full advantage of the 6gbps controller anyway (I'm sure there are a few higher end SSDs that might), and even if it theoretically doubled the access speed, when it comes to things like loading screens in mame we're talking a fraction of a second vs a slightly smaller fraction. The loading screens in Dreamcast, Playstation, Saturn, etc games also occasionally have limitations on how fast they can be accelerated by the emulator. Those systems had an expected disk access speed, and some games take this expectation pretty seriously. The emulator will only feed data into the emulated system's ram so fast, regardless of what your disk might be capable of, so you'd have to consider that on a case-by-case basis.
Specifically what board/drive are you looking at?