Given that you have the space, I would get the 138, to give you the option of 5x7, if you ever want to.
But you would need to have another enlarger for smaller formats. I use a Durst M600 for 6x6 and smaller and a L-1000 for 6x6 and larger.
The problem with both enlargers is, if you are missing parts it will be difficult and expensive to get replacements.
Durst no longer makes enlargers, to your only source for parts is the used market.
Get the manual for both enlargers and make a list of parts that you need to have and check with the sellers if the parts are there, or do an inventory in person.
It will be critical to get all the condensers for the enlarger. These can be hard to find.
Condensers are matched to the lens, not the film format. Example you can use a 80mm lens for 35mm film, instead of the usual 50mm.
For both the negative carrier, or negative plates will be important.
But correct bulb for both enlargers are no longer made. But as I understand, you can use the G40 bulb for the 138, and I think the G30 bulb for the L-1000. The G bulbs are not photo bulbs, buy from people doing tests, it seems to work fine. Here is the post about replacement bulbs for the 138. You can read the entire thread, makes for interesting reading.
http://www.largeformatphotography.i...placement-Bubs&p=816464&viewfull=1#post816464
I used the results of the post to decide on the G30 bulb for my L-1000.