There is a whole thread on stainless grates I'll post a link to. It's controversial. I, too, use my Weber Q a lot, now that the kids are raised, it gets the most use out of all my cookers for just the wife and me. But I don't find getting new cast iron grates every few years that onerous.
Here is a link to the Stanbroil grates, still fairly economical.
Here is a link to the discussion here:
I installed these 2 weeks ago, and they are amazing!! No more rusty cast iron for me. I have gone through 3 sets in 6 years, granted I grill a lot. Four or more times a week, and I kept them well seasoned. These are very high quality and work Fantastic.
tvwbb.com
These cast iron grates are highly rated, but less expensive than Weber OEM grates.
The Weber brand grates are $98 at bbqguys.com with free shipping.
The Weber 7645 grates are made from porcelain enamel which helps prevent food from sticking. The grates are split into two pieces, which allows you to remove one section and replace it with a griddle or other cooking accessory. For use with Weber Q 200 & 2000 series grills.
www.bbqguys.com
My current grates are the after-market type, not sure if the same brand as my first link, but about half the price of Weber and I like them. I do not clean my grates after cooking, I just leave food on them, turn off the regulator, then the gas bottle (I have a regulator for a full-size bottle). I just brush them off after pre-heating on next cook. No grate cleaner, no washing, occasionally, I'll brush the underside, but mine seem to hold up fairly well.
If you're only through two sets in 12 years, I'd say just get new cast iron. Seriously, how bad is 6 years for a $45 grate? OK, say 5, since it sounds like yours should have been replaced last year. I love the 9mm stainless grates on my Ace SA-E-330, but I like the cast iron for the Q better.
Good luck!
