First, an old mtb with slicks is never the practical choice.
High BB gives you good visability (esp. if you used risers - which I don't, but still), you only really need 1 gear = limited maintainace, light (ok not the visp), quick acceleration - you're always limited by lights/traffic, few parts to steel... etc... etc... etc.
Old MTB has high BB, risers, only one gear if you ghetto-SS it "... etc... etc... etc." and the most expensive part of the whole project is likely to be the tyres, since you can buy old steel rigid MTBs for peanuts. Doesn't matter how few parts your track bike has, it's still a bigger loss if you've spent £100 on a hub and somebody nicks the whole bike.
Old MTB has high BB, risers, only one gear if you ghetto-SS it "... etc... etc... etc." and the most expensive part of the whole project is likely to be the tyres, since you can buy old steel rigid MTBs for peanuts. Doesn't matter how few parts your track bike has, it's still a bigger loss if you've spent £100 on a hub and somebody nicks the whole bike.