Andrewg - I went through this same thinking, and am in the process of building it now. I'm using the spacer method, it's requiring 15mm spacers with the 20mm hiwin rails to get enough clearance with no pocketing. My thinking is that if I can get the CNC up and running I can then pocket out a new piece if needed, as I'm more likely to mess up the plate routing by hand (and no access to mill anytime soon). I'm offsetting the overhang by using 15mm thick tooling plate instead of 20mm. So saving 10mm but spacing 15mm.
Mine is untested so far though! - so I'd go with Jazz's and others advice any day if you are happy to pocket them.