OP, The question you ask is exactly what i looked into when I owned a D70Mk2. I was so impressed with this amp, that i was seriously interested in what the larger models would deliver.
The M100's, and I think if in good shape ( which applies to all amps of this vintage) would be my preference. The D series of this era are an absolute bear to bias...in fact I have stated here and elsewhere that it borders on criminal! This fact alone would sway me to the M100's. To that, I am a big fan of mono amps. The imaging of mono amps just sounds so amazing to my ears. While the D115Mk2 is no doubt an excellent amp, and will cost you less, for the long term, the M100's would be a no brainer, at least to me.