The Boss has a much wider range of compression ratios and is relatively transparent (it rolls off highs a bit but lows seem unaffected) without much tonal enhancement.
The Diamond has a fixed compression ratio that is fairly low (2:1 or 4:1 if I recall correctly) and it's real value is in fattening up to your tone while not losing any presence and IMO it does that quite well. I prefer thicker compression but I can see why guys dig this compressor.
I'd use the Boss if you want more versatility, need to tame big spikes (from slapping or envelope filters etc) or if you just want to save money.
I'd get the Diamond if you want a compressor to add some magic to your tone.