I understand (generaly) the differences, though I don't recall which is which. I had the cheaper (~$300) performed last time-round and want to know if I must pay for entire (~$1000) higher-level maintenance or shop ala-carte?
2002 E430 4Matic
Welcome aboard. Please click user cp and add your vehicle and location to your profile, it's useful information.
When you've done that, please come join the party in the W210 forum. This general forum is just that, general: you'll find a WEALTH of information about your car and your question in the W210 sedan forum. Once you get there please take a minute to read the new member announcement, and then peruse the help/diy stickies to get a handle on the kind of information in there.
Brief answer to your question: as a general rule, the major service done at A & B is an oil change and level inspection on other fluids. There is also a long list of items to be inspected (longer on the B service, and some dealers add some additional services to the B list). Can you do it a la carte? Sure. Heck, you can do all of it yourself, too, but it will take you a while. The inspections are IMPORTANT and can prevent catastrophic failures (like tearing a flex disc in two) or can catch things early when they are cheap and easy to replace (like a bad tensioner or idler pulley) rather than when they fail and leave you stranded.
So if you're not going to DIY, stop going to the dealer and find a good indy. A good one will do a better job at a better price than the dealer, and you'll go broke at a slightly-slower rate.
Good luck and we'll look for you in the W210 forum.