Field grade or service grade from the CMP?-that is really the question. I wouldn't waste any time or money getting a rack grade, because you're likely to get a real beater. That said, the answer depends on the reason you want a Garand and how you'll go about getting yours from the CMP. If you want a WW2 rifle, you're more likely to get that with a field grade nowadays. Although there are service grade WW2 rifles to be had from the CMP, I understand that most of their service grade rifles these days were manufactured after WW2 and, for the most part, after the Korean War, too. On the other hand, if you want a shooter, the service grade is your best bet. It will generally give you a better bore, a better stock, and a better metal finish. Especially if you mail order (or FedEx, as is the case with CMP), the extra $100 is good insurance that you'll get a decent rifle. On the other hand, if you can trek to one of the CMP's stores (in Ohio & Alabama), you might be able to pick out a plum field grade that will give you all you want at a lesser price.
The stocks and finish are not much different on the Service and Rack grades. The main difference is throat and muzzle wear.
Read the descriptions of the different grades on the CMP's website. Although a service grade will guarantee you a barrel with less throat and muzzle wear, I know there are field grades (but perhaps not rack grades) with barrels that measure up to service grade specs. On these rifles, the main difference will be the condition of the wood and metal. Specific to the rack grades, I have been to the South Store and can tell you that not one that I've seen has looked as nice as a service grade. Again, though, the best way to guarantee (as much as you can with milsurp rifles anyway) that you get a good rifle is to either order a service grade or go to the CMP store and pick out your own.
Get a service grade and ask for a new stock. They'll put one one no charge.
I don't think this is exactly the case. A lot of the service grades being sold now have new stocks because the CMP has experienced a shortage of decent wood for their higher grades. You can certainly request one of these rifles with a replacement stock, and the CMP will usually honor such requests, but know that it's not a sure thing.
