While it is true that the Mosin Nagant is a little more powerful than the .303, for all practical purposes, they do the same thing.
I have an M-39 and and SMLE. Of the two, I would take the M-39, but only because for me, it is that "one rifle". I have found my dream rifle, and it is a 60 year old Finn. It is deadly accurate, simple, durable, and tough. Mine also happens to be quite beautiful, with just enough meat on the rear end.
The "meat" makes the recoil so soft that you can shoot it all day and go home feeling great.
Having said that, the SMLE is one hell of rifle. It will lay some smack on you fast and hard. The action on an Enfield is as quick as you will find on any turnbolt, and with its 10 round mag and strippers, it is fast. The cock on close works just fine when you work the bolt hard and fast like it was made to be worked. If you imagine that there are angry Germans shooting back at you, it helps.
Both will reach out much farther than I have any business shooting and will make a mess of whatever they hit when they get there. I will agree that while I can find .303 within about 20 minutes, I can order 7.62x54 noncorrosive ammo cheaper. I can get 60 rounds of Wolf 7.62x54 locally for what I buy 20 rounds of .303 for. I can get 500 rounds for my Mosin Nagant for $100. It is true that you can't compare Wolf to Winchester, but both go bang and price is a factor. I would rather I don't know of any deal on .303 that will get me that much ammo for that price.
Unfortunately, I haven't shot my SMLE in a couple years because of the price of ammo for it, but I am going to use it to hunt this year.
I am planning to start handloading for both as well, so hopefully price won't be as much of a factor.