The two complement each other well so it's hard to say which you should use.
An EBW will massage but not manipulate the body and they generally can't tell you what the problem is if there's an underlying issue (unless they have additional training outside EBW). My boy has regular massage from an EBW and it's great for specific problems and "maintenance" between back-person visits (I use a McTimoney chiro) - for example, he had a few schooling sessions when he couldn't flex to the right and one massage found a tight spot behind his poll and he was fine again. The EBW keeps a record of each visit too, so she can compare the tight areas over time and see if there is improvement / any new stiff areas developing. Once they're used to it, horses tend to position themselves so they can be massaged where they're sore!!
EBW is usually cheaper so it may be worth doing that first. If nothing else, they'll be able to find out where any particularly tight areas are and you can pass on that information to a chiro / osteo.
I have both too, I normally have the EBW for a 6 weekly treatment just to help any stiffness/niggles then have the osteo every 6 months for a MOT normally at the begining and end of the season.
I find this works out very well and they compliment each other.
Definatly reccommend the 2aswell, i'm a equine massage therapist, i do a friends horse every 6/7wks, he's 19yrs, a bit arthritic in his near side shoulder, my friend has commented that after each massage session he feels more supple and flexible in his movements, he finds it easier to do whats asked of him in his work. And by muscle testing i can detect and ease out any spasms/cramps or tension in the muscles.
If you've ever had a massage yourself, then you'll understand how this can benefit your horse aswell.