That is a tough call. I think both colors are stunning - especially in the First. If I had to pick one over the other I'd go with Grenat just because I absolutely am ga-ga over that color. I have a Grenat First and it's like a glass of wine you want to savor for hours. YUMMY!!!
If a rare pumpkin First came up for sale on ebay (as it is right now) I'd get it just because it isn't made any longer and Grenat is much more readily available in every style.
The pumpkin on eBay used to be mine It's a gorgeous bag that goes with everything. I would almost dare to say that its a neutral since IMO it's very close to a caramel/carmel. It really depends on your preference though. Grenat is in a very different family of colors than the pumpkin and is a lot richer. Both are excellent choices to pick from!