What are you using it for? What are you playing. I've only tried the Cube 30 and Spider III 75w. Between those 2 I'd go with the cube. The Cube was much more natural sounding and not as "sterile" as the 75 in my opinion.
I have only used a spider 2 and know I use a roland cube 20 x. I find the cube a lot warmer sounding and the power amp sqeese feature is brilliant. On the other hand though, I have had vetta 2 hd 157 boss gt 8 and a shitloads of tube amps so I know what I like from an amp. For portability and very good bass response the cube 20x is my weapon of choice. It,s simple to use and the tones are very nice for a digital unit.