The Garand, besides being an excellent rifle is a piece of American History. If you can, get one now from CMP. They wont last forever.
Ammo from CMP is reasonably priced also.
The Garand is easier to shoot. Ever try to load stripper strips in a Mosin. It can be done but takes lots of practice.
The sights on the Garand are the best ever put on a military rifle (M14s have the same sight). Most Mosins shoot high, and will take some work to get them set right, even then you don't have the ability to adjust for windage. You can the Garand.
As to 1903a3. Yes they are Great rifles, I think the best military bolt guns ever made. However, good ones are hard to find. In the non-modified mode you'll find the prices higher. Not sure they are more accurate then the Garand, I'd say they were the same in the accuracy department. I get that from compairing the Scores fired in the CMP Garand and Springfield Matches. The scores are pretty much neck and neck.
You wont find a Mosin that competes with either.
If you deside to shoot the CMP GSM vintage military rifle games, the Garand and Springfield have their own matches. Mosin's have to compete with "all other vintage military rifles". So you have to shoot agaisnt the 1917 Enfields and other rifles such as the Swiss, Swedes and Mausers.
I like Mosins, I have one and after tinkering with the rear sight I got it to shoot point of aim at 100 when sent on 100. Tons of dryfiring and shooting its gotten pretty smooth. But nothing compaired to my 1903a3.
All rifles are going up. GI AS Issued rifles are going up more then other rifles simply because they arent making them anymore (the CMP just upped the prices on their Garands). Mosins are cheap, they will go up, but have a ways to go before they are out of reach.
I'd get the Garand now, go for the Mosin later. And if you ever find a good, AS ISSUED Springfield and/or Enfiedl, go for it. They are getting rare.
As an example, I put off getting a CMP M1 Carbine. Finely I saw the light, I was schudeled to go to the CMP GSM MI course in Sept 2010. I was going to pick up a Carbine when I was at Perry for the course. But in June that year I desided not to wait and ordered the Carbine..............in Sept they were gone, NO MORE, except for some expensive examples found on the CMP Auction Sight.
When I got my Garand from "then" DCM, It cost $112.70 delivered to Anchorage. Now the Field grades are $595 + 24.95 shipping. The prices will never go down and they arn't making any more.
If you buy a Garand, I'd recommend the CMP Garands. All CMP Garands are inspected by CMP Armors before they are put on the shelves. PLUS, if for some wierd reason you have a problem, you'll never find any better customer service the the CMP.
Also, the profits from the sales of CMP rifles and ammo go to support the CMP Marksmanship Programs geared mostly to Juniors. That in itself is a great reason to choose CMP rifles.
Remember, besides one of the best military rifles ever made, you are buying a piece of history. There are still millions of Mosins out there you can get later.