if yer a fan of high revving 4-cyl engines, the M42 a great choice, and has some more modern conveniences (hydraulic valves, chip-able, vs. the M10 which is not).
Building an M10 to S14 specs/power is often nearly as expensive as getting a new S14, and although it has a lot of potential b/c of the block, there's some work involved.
on the flip side, b/c the M10 was in the 2002, E21, and E30s, there's a ton of information about building it up.
For me, I wanted to avoid a lotta fuss and keep the dynamics of the car the same, so building up an M42 worked out well for my particular goals / plan. Also, the dasc supercharger is now a solid, viable option for making good bolt on power.