For a 50W mercury vapor, should I use a 50W metal halide ballast or would 39W be better?

I figure I'd have to go with a transformer ballast so I could remove the igniter. Is it obvious how to wire it up once the igniter is removed?

Has anyone been able to remove the igniter in a metal halide electronic ballast cause they are much cheaper and I like how small they are.

Personally the best option for me is to get a 50W MV ballast... 

Always best to use the correct gear if at all possible. Sometimes, certain ballasts are virtually impossible to obtain, such as for the 200W SLI/H lamp, but if you can get the correct one, do 

I think the metal halide 50 W lamps have a somewhat lower arc voltage but higher current than 50 W mercury lamps. Since the ballast mainly determines the current, using an MH ballast for 50 W will cause some overdrive. On the other hand, a 39 W metal halide ballast will cause some underdrive. Between the two, I would pick the lower one since I think it is probably less harmful for the lamp.

If you are in "230 V-land", you can parallel a 40 W fluorescent choke with a 7/9/11 W PL-S choke. I have operated 50 W MVs like that and the final current ended up at the specification (0,61 A in my notes).