Gaz,
As ever - imho it depends! Roobo gives some good advice and source of further info. Below is my thinking on your question.
If you have no other grinder at all I would suggest a dry wheel grinder with a jig.
Dry Wheel Grinder
Budget is the next consideration:
- Low - basic 6" grinder with white wheel and home made jigs.
Medium - 6" grinder with jigs (tormek jig on mount / wolverine jig / sorby / axmister storme v.good value)
High - Slow speed grinder (crusen rolls royce machine) with jig as above.
With all of the above a 'white' wheel is the basic minimum and all you need to get going. Other wheels are available that all balance cooler / better sharpening vs softness / wear rate / cost but leave that for another day.
Sorby Pro-edge machine an option with pros / cons and has been reviewed on here several times - I've never used one so can't comment from direct experience so will leave others to take the floor on this.
Wet Wheel - tormek / jet etc.
Very good for touching up an edge but u/s at re-profiling or grinding initial shape (slow and stone wear). I would see these working along side a dry grinder as a second purchase if you felt you needed it rather than a only option. They do need the associated jigs which can push up the purchase price significantly (note jet and tormek jigs are interchangable. The tormek gouge jig is the best by some margin, the others are much of a muchness in terms of function although the tormek stuff generally feels better engineered to me).
The other thought will be what other use you need to put a grinder to. If you are a carver or cabinet maker than the wet wheel solution may have other benefits, if it is just to give a log splitting maul a quick dress each autumn then the dry wheel will surfice I would suggest!
Final option is to mount an abrasive disc on the lathe (outboard if you can) and use that with a homemade rest of some kind. Many do to really keep start up cost to minimum but most move on to some sort of dedicated grinder in time.
Hope this helps,
Simon.