Not really. Deciding which one to buy mainly depends on your style choice and what you generally plan to use it for.
The rifles id say are the best at doing everything well. They are the most compact, lightweight and better balanced of all the versions and they have the greatest aftermarket accessory support. They are however the most likely version to come with misaligned trunnions and loose rivets. They dont have a threaded barrel, the fixed stocks are often considered to be the most uncomfortable of the Type 81stocks and the sight radius is the shortest.
The LMGs are larger and much heavier. Understandably they are fairly front heavy and only pleasent to shoot in a position where the bipod is deployed. Due to its weight, it is the lightest recoiling of the versions to shoot. The clubfoot stock is widely accepted to be the most comfortable of the Type 81 stocks. The sight radius is longer allowing for more accurate aiming. However the bipod reduces accuracy to a point where the system IMO groups the worse out of all the other Type 81s. The bipod however can be removed and so can the carry handle. Stripped of these parts, the LMG essentially becomes a slightly longer, front heavy Type 81 rifle layed out more similar to an AK. Combined with a great amount of aftermarket accessories and the threaded muzzles (M16x1 or M14x1), the stripped LMG becomes a solid choice. The LMGs are likely to have much better aligned parts then the rifles, however they are also more likely to have factory rust.
The SR is essentially a stripped down and redressed LMG. Notable changes being the chopped rear portion of the receiver to allow for a thumbhole stock to be equipped, a LMG profile barrel with different profile steps to locate the gas block and front furrule further forward, and the removal of ugly steps to support the LMG bipod and cleaning rod retainer. As a result the forend is notably longer then the other Type 81s, the SR has also had a trigger overtravel stop added which provides minimal improvement in trigger operation. The SRs like the LMGs have better aligned trunnions however visually misaligned gas piston covers are a common issue. Like the LMGs, they are also more like to have rust from the factory. The SRs come equipped with a gen 1 stock that is very prone to breaking, a gen 2 that supposedly doesnt break is included in the box. The SRs recoil similar to the stripped down LMGs (nicer then the rifles). Its muzzle is threaded in M16x1mm. The stock comes with a detachable cheek pad which is nice for using optics. It currently has the least amount of aftermarket accessory support. It always comes with threaded fire control groups pins and a right hand safety ("optic ready" configuration) however the needed side rail to equip optics is often not included.
All Type 81 versions have the same reliability and debatebly have the same accuracy potential.
I personally prefer the SR and stripped down LMGs as they feel more natural to shoot beyond 200m. And personally find the SR to be the most comfortable (a highly disputed opinion as many find the SVD style stocks uncomfortable).
I own all Type 81 versions.