PS2 or PS3???

I would go with PS3 if I had a choice, no battery and better sound for starters. Also newer technology, which I always like. Would imagine parts and things for PS-2 will become harder to get at some point in the future, as have other similar things that were updated with newer stuff. I know of no disadvantages to PS-3?

Edit: And as Matt added, LED's, forgot that one.  Also, I thought I read somewhere that MTH was supposed to be upgrading the PS-3 sound files?  Matt?

The only real difference between PS2 and Ps3 is the PS3 has a DCC decoder on board, PS2 uses a battery and bulbs for lights PS3 eliminates the battery by using super capacitors which is nice being that batteries die over time and then locos dont work and PS3 uses LED's for lighting. There are some other minor technical differences but otherwise the two aren't much different as far as O gauge railroading in concerned. They both run on DCS and the sound files are essentially the same

I did notice that (led lights) on my PS3. And that headlight is a HeadLight. It will shine ahead on the track where the others are just for show. Of course you have to watch it in the dark, when that sucker comes out of the tunnel, and you are down at track level and it hits you square in the glasses..............................

I would stay away from the PS-2 5V systems unless you get them at a steep discount.  Some 5V are prone to failure, and the boards are older.  2000-2004.

I wouldn't hesitate getting a PS-2 3V though.   G

I am correct the 5V systems had the 8.4V battery?

Yes, and round charge port if it had one.  All the PS-2 3V have the rectangular Molex style charge port with 2.4V battery, though a few very early models used two independent AA batteries.  G

I'm with GGG here, I pass the 5V models by unless I'm getting a REALLY good deal.  That way I can justify replacing the board if it goes.  I've already had two of the 5V boards in my own stuff go out of about a dozen engines, but no 3V boards have had any failures.  I also have a number of 5V equipped units lined up on the bench for repair waiting on replacement boards.

I would not hesitate to buy a 3V PS/2 unit, those have performed very well for me.

After going back and looking up the PS2 numbers, the cheaper ones were the early 5v boards. The later 3v were basically the same price as the PS3. So I guess it's just which engine will strike my fancy. I'm somewhat limited because I have some 027 switches and track on the lower level. My AS616 requires 031 but some careful trimming of switch covers fixed the small clearance issue. So I'm using the length of that engine as a guide as to if the engine I'm looking at will make it around the lower level. Some such as the SD70ace says 031 but it's almost 2 inches longer than the AS616. Thanks for the use full info. At times I might have been inclined to cheap out, save 100 bucks and live to regret it later.

You are pretty limited in what you can run if you're limited to O27 compatible stuff.  That's the reason I decommissioned my last layout, couldn't run anything big on it.