Separate pump, or all in one unit?

I'm planning to refurb two upstairs bathromms and want to boost the shower pressure that is currently rather weak gravity fed. What factors should I consider in choosing between (a) box-on-the-wall units with integrated pumps and (b) a separate impeller pump feeding both shower heads? I have a normal (non-combi) boiler, cold water tank in the loft, and HWC next door to one of the bathrooms.

Thanks

Pump in the box units usually have more than enough pressure for most people and cost to buy about £140 from focus etc which is cheaper than you will get it from a merchants.. They are generally quieter in operation than a seperate pump unit and on the whole they are very reliable. Newteam where the biggest manufacturer but have been taken over by bristan