Chrome-Moly steel (a steel with a high chromium content) is an alloy used to make most gun barrels among other things. Another popular metal used would be Stainless Steel. Chrome-Lined means they literally line the bore of the barrel with chromium.
So basically your choice is Hard-Chrome lining or not (yes, these companies should just ask it that way and make life less difficult for the uninformed).
For $45, I'd say go for the chrome lined bore (you can't add it later if you change your mind). It's harder than the chrome-moly steel of the barrel, more resistant to corrosion and easier to clean after shooting. The only drawback is it could be slightly less accurate, but not anything 90% of the shooters would notice. I have some really accurate chrome lined ARs.
DnPRK is right though, an SS barrel would be a happy medium between the two if extreme accuracy was your primary goal (like for a long range varmint rifle or a competition rifle. These rifles usually have SS barrels, most often 20" or longer to keep the velocity up for longer range)