Buying DT225: Serial for a lower price or pay more for PS/2?

[Synnöve]

I've decided to purchase a DT225 as my research has borne the conclusion that it's one of the smoothest, most reliable trackballs out there. I have the choice to purchased two varieties of DT225. Here are the facts:

-One seller has serial cabled units, the other has PS/2.
-Serial seller has new units for 75 and used units for 50. Reports that trackball rolls smoothly. Sells directly from their industry supply company.
-PS/2 seller has used units for 65 (Includes a Cherry ML4100 as well), reports that the ball does NOT roll smoothly, sells via ebay as a PC parts liquidator.
-According to the user manual, the serial model can use the Microsoft 2 button protocol or the Mouse systems 3 button protocol.
-In either case an adapter will be required.

I really want to purchase the PS/2 model as it not only comes with a free KB but it also looks better in terms of it's aesthetic condition. The thing that puts me off is the seller claims it doesn't roll smoothly. Could the bearings be worn out? The roller shafts? Could the ball have bumps? I'm not sure.

The safest option is to purchase a NOS serial model but it does have quite a price premium and the serial interface gives me pause as I imagine it'd present even more compatibility problems vs. a PS/2 mouse.

Does anyone have some advise on the matter? Is it likely the bearings on the PS/2 model just need cleaning? Will the serial model be fine with the proper adapter? All suggestions appreciated : ).

[Tactile]

They might not be the same resolution. If that's important to you, I think the serial number contains a code designating the resolution. A document which might help with that is here:  http://www.apem.com/files/apem/brochures/DT225_JOYST1405R1US.pdf

I don't know what kind of bearings they have. I had an Itac trackball in which one of the sealed ball bearings failed. One of the balls in one of the bearings shattered. The ball would roll perfectly smoothly most of the time, then, if the bad bearing got to the right spot where a bit of the shattered ball was, it was like rolling over a pebble - or sometimes that shaft would stop dead with the ball sliding on the shaft rather than rotating it.

If they do have ball bearings, the ball doesn't glide like, for example, a Kensington Expert Mouse. I have Evergreen & Itac trackballs here & their normal operation might seem a bit rough to someone not used to them. In other words, it might be fine.

I'd go for a PS/2 or USB for myself.