Hi,
really does depend on what you are doing with the car, in real terms if you are staying fairly standard changing from top mount to front mount you will increase your tubby lag but not much else, whilst a front mount in theory is better cooling hence morepower, the lag counter acts this, you will read about heat soak with the top mount, I never experienced it but that doesn't say it doesnt happen, in fact I got the symptom of heat soak with the fmic but was something else.
If your intention is to modify in stages, then this is one of the earlier mods, however if the engine is coming out, spend your money uprating engine, clutch, gearbox while out, ie. everything which can only be done out of the engine bay, will save you money later as these as standard will hold you below 400bhp safely, do lots of reading on here and other sites, learn what the car is capable of with standard engine and gearbox, if that is as high as you want to go then front mount is one of the first options.
fmic compared to tmic, realisticaly +20bhp others might disagree, however they differ mainly on max BHP they will handle
cost £350 - £1200 dependant on make
were to buy - depends on how deep your pockets and how you want the car to look, otherwise easy to fit.
whilst the above is quite general, lots of research first is the best way to go forward, save you money in the long run. I run an ND stage 2 fmic because my main criteria was that it had to fit with the ac and keep front of car looking the same with the capability of 450 bhp, I run 360 at the flywheel but after the next set of mods will be seriously thinking of dropping the engine again to uprate,
regards
Nigel