I'll start by saying this isn't intended to be rude or knock you for asking questions, but you don't seem to have a grasp on what you have, much less a starting point for technical questions. that said I'll do the best I can.
To start, I'd read through the manuals provided with the engines, they are usually written at a level a child can understand and will walk you through the functions of the engine. There are plenty of books and this forum has a search function that may be helpful for the basic questions.
On the speed difference of the original post, MTH with the DCS system uses scale miles per hour as it's speed system. the numbers count from 0 to top speed in one mile per hour increments. Lionel uses a different system with their TMCC and Legacy engines. we'll just address Legacy here, since you don't have any TMCC from what you said. Legacy uses 200 speed steps from 0 to the top speed of the engine. these steps have no relation to miles per hour. They don't quite work out perfectly evenly, but think of each step as 1/200th of the engine's top speed, or maybe about 1/2 or 1/3 mph per step.
As for cruise, all Legacy engines have it built in. I think all PS3 do as well, and many ps2? I'm afraid i'm not very well versed on MTH products since they don't offer features I require in a control system. Some TMCC engines have cruise but most did not, while most aftermarket upgrades would have it.
One last thing, you mentioned a 1998 engine with PS3? is this an upgrade kit or ? PS2 wasn't out in 98, never mind 3.
