The HD grills have either porcelain covered cast iron grates or cast iron grates. Neither is bad, just not stainless.
The difference between HD and Ace is (SHOULD BE) service and support, but Ace will almost certainly be significantly higher in price.
At HD, unless you should get a damaged grill (unlikely) you are done after you pay your money (for a Weber grill, an extended warrantyis a TOTAL waste). You must take care of any future issues and contacts with Weber on your own -- not a difficult proposition for many. If you are not "handy", you may prefer the Ace route, but you should seek inducements (free accessories) and you should get a lot of attention including free assembley and delivery. If you don't get that, find another one (my local Ace was a dog!).
The Spirit is a really good grill -- it's backed by an outstanding Weber warranty. How much you pay and what you expect is up to you.
Rich