Hi Henry,
When I decided to purchase the PM25 I thought I would sell the LMS, but have decided to keep it instead. I've never had a second/backup machine, but I have already made parts for the PM on it's little brother.
The reason I bought the PM was because I wanted a slightly larger machine. I REALLY wanted the PM30L, but I wasn't sure I could handle it by myself.
The biggest advantage the PM has over the LMS is size of course. While I don't have farm implements to repair, or need to make custom parts for a hot-rod/Harley build (I wish...), I found the LMS a bit on the small side for some things I do. The PM is literally the LMS' big brother. The LMS weighs under 150 lbs with a vise and DRO scales on it and I can pick it up and move it by myself with no hoist or other assistance.
The PM weighs almost double that, and I bought a one-ton shop hoist to lift it onto the tool box. Can't quite pick this guy up with my bare hands. As most of the Big Iron guys here will tell you, bigger is better. Mass really does make a difference, if you can afford or have the space for it. I have neither, and honestly, wouldn't begin to use the capability of a BP or a 14x40. But...having twice the weight/mass behind a 1/4" or 3/8" end mill makes a world of difference.
Advantages/disadvantages? I guess this boils down to opinion, but I'll list mine and you can decide for yourself if any of it is worth a hoot.
LMS:
Small(er) footprint, yet capable of some very accurate work as long as you stay within it's intended work envelope.
At first I hated the rack gear on the Z-axis, as it can move on you while you're cutting if you are not careful. But after installing a 3-axis DRO and the Air Spring, that became more manageable. Now I have become accustomed to it and actually like being able to 'rapid traverse' the head up and down when I'm changing end mills or drill bits.
20 TPI leadscrews on X and Y axis. These allow for very small precise movements. Once the DRO was installed, I came to realize just how small/precise. Of course you have to crank your hand off to get from one side of the table to the other. Tradeoffs...
PM25:
Primary advantage and principle reason I bought it is the increased table size and Y-axis travel. I will soon have need of doing some work on a rotary table and wanted a 6". This will fit SO much better on the PM's larger table.
Larger work envelope. While the LMS can do 'most' everything I need, it will be a little easier on the PM. My max work envelope is usually about 4" square, but can sometimes get to 6". 6" gets a little tough on the LMS with it's shorter Y-axis travel.
Power and mass. More is better. Even though I have no need or desire to bury a 1" end mill in a 12" square piece of 4140, I do appreciate the smoother cuts and less vibration I get with my 1/2" and smaller end mills. There are limits to what this machine can do, obviously, but for my pistolsmithing and other projects I am well within the capabilities of this machine as long as I don't get stupid. I knew I was borderline with the LMS, but didn't know about the PM/G0704 types at that time.
The PM has courser leadscrews, so real small movements are more Art than Science, but I am putting a DRO on so I will figure it out. I like having a leadscrew on the Z-axis from a control perspective, except when I need to rapid traverse. I have this vision of putting a stepper motor on it with a manual digital control wheel and a course and fine feed rate. Like my friend's CNC control. Maybe the guys in the CNC forum can help me figure out how to do this.
That's all I can think of at the moment. If you have any specific questions I will try to answer as best I can. I guess it really comes down to what you want to do and how much machine you have room for.
Bill