There are major differences between a M/94 and a M/98. First, the action name itself is not a M/96, 'cos this refers to the long rifle, with a straight bolt handle. The first commerncial rifles made by HVA where based on brand new and fresh M/94 actions, made by Carl Gustav. Then, CG stopped making these actions and the manufacturing was brang to HVA works. For the military, they developped a mid-sized rifle, called the M/38, wich used the bent handle of the M/94 and a 24 inches barrel (instead the 29 inches of the M/96).
Now, HVA used the M/94 on the pre-WWII models (model 46B) and the early post WWII (Model 646) where based on the M/38 made by HVA, but with what is called a "solid wall", without the thumb cut-out for the stripper clip loading. These actions are small rings.
HVA also made the later (late 1946) 646 with the FN-made M/98 actions. These actions are large rings.
Then, you have the HVA "improved mauser", the 1600/1640, wich is a cross over between a M/94 and a M/98, retaining the features of the M/98 (three lugs bolt, ####-on-open, bolt shroud) and the bolt head / small receiver ring of the M/94.
Now, when it comes to choose wich one you want, you have to take consideration that the M/94 was designed before the coming of high power rounds (3900 bar or 57 000 PSI of the 8X57JS, in 1905) and that the M/98 was designed with their use in mind; Paul Mauser knew it was coming, and that's why he increased the "safety factor", the escaping gas beyond others, of it's new M/98. But this action is a bit heavier than a M/94-96-38. The 1600/1640 is about the same weight as a M/94 but will handle much better the fury of higher pressure rounds, 'cos it was designed to do so.
If you will use factory loads, of stay within the reloading manual loads made for it, the M/94-96-38 will do just fine. If you want to push the beast to "modern" levels, then, you will need either a M/98 or a 1600/1640.
Just remember the military 6.5X55 was adopted in 1894, with a heavy and slow moving projectile (at lower operating pressure), then changed in 1941 to a higher velocity lighter bullet (140 grains), operating at a pressure of 3300 Atmosphere (48 500 PSI).
My personal choice would go for a 1640. For a light, handy rifle the early M/646 based on the M/38 ("commercial") is a good, economical choice. The M/98 based is the strongest, but the heavier one too and the 1600/1640 action was designed to handle modern magnums. For collecting, any of these is of great value, especially if not altered.