E38 pros : not susceptible to rust like the W210 is. cheaper parts. more DIYs available than the benz. better looking IMO.
E38 cons: beware of the transmission. the ZF 5-speed autos are iffy and its lifespan is limited to 80-100k if the fluid has never been changed, some have had theirs go for 200k so its possible to get long miles if you change the fluid every 50k or so. E38s also suffer from the crappy cooling systems so figure that in as well. front thrust arm bushings are common wear item. plan on replacing those every 50k if you plan on keeping it for a while. valley pan gaskets and valve cover gaskets leak too but its DIY. parts arent horrible either. the M60/62 is a good, solid engine if properly maintained and can go over 200k easily.
W210 E55 pros: rare, not many E55 AMGs were imported to the states, super reliable engine and its DIY friendly. No supercharger and heat sink issues to worry about. looks good as well and you are more likely to get most of your money back on this than a run of the mil E38.
W210 E55 cons: rust! when you go to look at one make sure the inside bottoms of all doors arent rusting through and look at the front fenders as well as rear wheel arches. the electrics can be shoddy too. the transmission seems to be a common failure item on these cars around higher mileage and it isnt cheap. i remember reading around $5k for a new one. the brake parts are expensive for these as well.