Well, as an Norinco M14 owner, and have never used an SVT-40, I have a bit of a bias.
The M14 isn't without its faults, but they're relatively few. My mistake was mainly with modifying the stock in an effort to make it go from 3 MOA to maybe 1-1.5 as I read was quite feasible with the rifle. The 'accurizing' made it much worse, unless you're buying an aftermarket stock, just leave it alone, it shoots great straight out of the box. The front sight blade is wide, it could use some narrowing, but that's it. The sight base on the flash hider is cast slightly out of line, so there's a couple degrees of tilt of the front sight. A new flash hider/sight would fix that, but that's the only manufacturing defect I've found so far and I've stripped it down to it's elementary pieces.
As I've read about the SVT-40 is that they're relatively inaccurate. They also don't have the chrome-lined barrel the M14 has. Both guns have an awkward scope mounting situation, but in general the M14 has far more aftermarket support.
I know the cheap milsurp is an incentive, but there's also cheap .308 milsurp on the market (Budget Shooter Supply, red link at the top of the page). It's not as cheap as the x54 milsurp, but it's still a bit less than $0.50 a round, and also corrosive. I have a case of it and I'm not sure I'd recommend it anymore. It's hot loaded, which I prefer to under-loaded, but having to clean the barrel and gas system after every shooting session is a pain. The chrome lined M14 barrel will make it last longer. The surplus SVT-40, has been used, and even if it hasn't already been shot out and corroded by millitary use and lack of stringent cleaning, it will corrode quicker, and I doubt finding a replacement barrel for it would be easy.
The x54R cartridge does have more power behind it than the .308, but I don't think it's worth the disadvantages. It's generally accepted to be an inherently less accurate cartridge, and you would be hard-pressed to find a backwoods hardware store that carries x54 ammo, in the event you forget or don't bring enough. The ubiquity of .308 ammo in various loadings, inherent accuracy, and still being a standard issue NATO cartridge are definite advantages. Rimmed cartridges like the x54R and .303B, while they can be accurate with the right gun, have become obsolete and there's usually a reason behind that.
I would avoid the .308 milsurp for the annoyance of it's corrosivity, but I would also avoid the norinco steel cased, berdan, non-corrosive. The cases can't be realoaded, the corrosive brass, can be converted from berdan to boxer and be reloaded. All that aside; PRVI and S&B both have inexpensive, boxer primed, brass ammunition, going for ~$0.70 or less a round. I wish I had gone that route, but I'm stuck with doing tons of cleaning until my crate runs out.
Summary:
M14: more accurate, ubiquitous (and still being issued) ammunition, chrome-lined barrel, more aftermarket support. Brand new, with 21st century metallurgy.
SVT-40: neat piece of millitaria, powerful cartridge, less accurate and hard to find ammo, but cheaper milsurp ammo, less aftermarket support. Old, millitary issued, possibly worn by high use and poor cleaning.
All things considered, if you want a utilitarian shooter, that you can depend on, get the M14. If you're a milsurp collector and have a specific hankering for an SVT-40 despite the shortcomings, then get an SVT-40.
Personally if I wanted a x54R gun I'd get a Mosin-Nagant; cheaper, easier to clean, bolt action is inherently more accurate.
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