Assuming you have equal passion for each of the ideas, I think you should choose the one which you feel:
- has the biggest possible market - has the lowest costs (e.g. maintenance, support, marketing) - is easiest/quickest to implement
- has the biggest possible market - has the lowest costs (e.g. maintenance, support, marketing) - is easiest/quickest to implement
for example, a smaller market with better acquisition strategies could be better than a larger market with nebulous or not unique acquisition strategies.