40g or 55g DIY sump for 125-150g tank??

Either is fine. I’d prolly go 40breeder since it has a bigger footprint.
 
15g is going to be negligible. The larger footprint will allow for more options in skimmers.
 
Agree with the bigger footprint. I have a 40b and constantly wish for more footprint! Also, isn't the 55 pretty tall? I definitely wouldn't recommend messing with that...

With that size tank, maybe even a sump and separate refugium. I can only dream : )
 
The only problem you might run into is fitting the 40breeder under the stand if the tank is 18” wide
 
My DT is 36x18, so I made the stand 48x24 (ima upgrade some day!!!)

BUTTTTT.... keep in mind that the 40b is 16” at the glass (or, at least mine was) - the plastic trim is closer to 18”. I ended up having to notch my vertical 2x4s so the top/bottom trim would fit between them :/ YMMV, but it’s worth checking first...
 
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