A 347 will be newer with more capacity, but in the end they are the same design. If your only looking at a couple thousand bales a year the 336 will be just fine, if more than that i would look at the 347. Price is also a factor. In my area a 336 can be had much cheaper than a 347. If they're the same money i'd go with the 347.
Just my 2 cents,
Casey in SD