Hmmm, good question. My take..............................
On a layout your size, a Lionel ZW-L is way overkill. It will power your entire layout and do everything you want it to do, and not break a sweat. You can throw your current little transformers in a box, stash them under your layout, and forget about them. Chances are good that you would never need another transformer the rest of your like. Plus there's the "cool" factor - nothing screams "Lionel" or "3-Rail" like a ZW-L does! The only downfall - it's almost twice the cost of a Z-4000.
On the other hand, for a layout your size, an MTH Z-4000 would be above average. It will power your entire layout and probably do most everything you want it to do, but may break a sweat occasionally. Better keep your current little transformers handy, in case you need a little extra oomph for something or other. Chances are good that you might need another large transformer sometime down the road, or maybe not. Fortunately, since it's almost half the cost of a ZW-L, that would allow you to buy more power down the road if you do need more. At which point in time you will have just as much money (or more) tied up in transformers than if you had bought a Lionel ZW-L to begin with.
After considerable research and sole-searching a few years ago, I sprang for a brand new Lionel ZW-L instead of an MTH Z-4000 for these exact reasons. As of right now, the ZW-L is the single most expensive O-gauge item that I have purchased, and I'm entirely happy with it! 
