Unsure about the 06-07 SPK shell, but with the more recent ones i have a feeling that the sizing runs larger or smaller (can't remember, and i don't wear the boot)..so don't commit to just going down one shell size in the dalbello, because that may not be right for you..
I'm sure someone with SPK's can shed some light on the sizing, it's just not coming to my head right now..
The kryptons are 8mm narrower than the SPK, so if you were going to drop a shellsize AND going to such a comparatively narrow boot, then you would want to make sure the krypton is right for you..I'm just saying this because they are possibly the most incomparable boots out there in terms of fit. Thats why I wondered if the SPK's were seriously wide on you - because they should be if you are looking at going a shellsize down in the krypton..also need to look at things like the volume of your foot. The SPK can handle a foot which sits quite high in the boot, but the krypton is quite low profile through the instep
It goes without saying that you should try before you buy
But hey, if the boot fits...