I have a Midjay and had an SD1 in the past; the SD1 has a nice 76 keybed which is not heavy at all; however, if you are after a super-light controller, I recommend you the Roland A-37 (I own that one as well), which is really extra-light (it doesn't even have an internal transformer and runs on batteries or an external transformer, like I have). The action is nice and, together with the Midjay, gives you a lot of power.
The only limit of the Midjay is that, being an all-in-one machine, the arranger section is not as programmable as in the SD1, so you have limited control on your styles, meaning that can edit them only to a slight degree and cannot program new ones.
Aside from that, the sound is good (just like the SD1) and it has plenty of possibilities I still have to explore (like the DJ loop playing ability, which looks very promising on paper).
Final consideration (very important to me):
- Roland A-37 weight: 7.7 kg or 17 lbs
- Midjay weight: 3.2 kg or 7 lbs
- Total weight: 24 lbs!