If you compare the magnabooster and the magnaclean the two products work in completely different ways. The m/c attracts wet sludge so you have a lot more evidence and you get chunkier bits of steel coming out on the magnet. The m/b has a spirotube (basically makes a vortex) which the dirt falls down through the centre of. It also has a filter that breaks down the sludge as it passes through. So, when you clean a m/c you see lots of lovely sludge, when you clean a m/b you get very fine powder that is hard to see. The 'magna' part of the booster is something added by spirotech on behalf of british gas basically as they used the magnacleans in the past. Plus you can show customers with a demo jar how the magnet works, whereas in fact, the spirotech filters work well without them. Spirotech make industrial filters and have done so for years, quality bits of kit in my opinion. When I spoke to their rep, they'd had no warranty returns and offer a 20 year guarantee on the product, Adey offer 2 years on the m/c, the seals fail and the lids pop off...
Filter on system is good, won't fix anything but is a good long term preventative measure. As others have said, if the system's sludged up it's pointless, get it flushed then fit one.
Or if the boiler's old and parts are unavailable and you're thinking about replacing it, get it done with the new boiler.
I'd advise checking how well the radiators heat up as to work out if they're blocked or restricted and then if they work ok, go for the booster... just my opinion anyway