ASRs are the best for chopping...plus they sound nice....warmer and fuzzier than Akai if you ask me...
I also have an s950 for its "crisp" sound...
If you want more extensive sequencing and mixing abilities id suggest a laptop, audio interface and program like Acid, Cubase or Pro Tools. That way you dont have to just sequence loops...and further down the line you might want to add vocals or instruments so a mic would come in along with maybe synthesizers etc