I've got both the CV15mm (LTM) and the Zeiss 18mm on my M8.
The 15mm is steal, especially considering that you get a perfectly fine viewfinder with it. You can drop a reversed 39mm filters in the hood and block it with a bit of tape so the lack of filter thread is not really an issue. I coupled it with a LTM-M converter with 6bit coding holes and coded it as a WATE.
The 15mm is so small that there is no reason not to take it just in case. It is my smallest and lightest lens.
Note: I use the stock 15mm vf with it. I found that by holding my eye not too close to the vf, the field of view was actually very close to being correct on the M8.
I recently purchased a Zeiss 18mm because I much prefer the equivalent focal length of 24mm on the M8. Haven't compared IQ yet but the lens is significantly larger and heavier, which means I'll possibly keep the 15mm as a carry-everywhere option.
My ZM 18 has a 28/90 mount and has been hand-coded as a 21mm 2.8 pre-asph, which seems to be pretty effective.
I use it the CV 25mm VF I got with my used ZM 25. 2.8.