You have some homework questions to answer before making recommendations:
1. Weighted or unweighted keys?
2. Mod and pitch bend wheels or joystick?
3. Want sounds on-board or not? in other words, will you ever gig with it by itself?
4. Do you use anything but the SD2 for sounds - like VSTi plugins in your PC? If so, it can be advantageous to have non-key controls available on the axe like pads, knobs and sliders that you can map to parameters in the plugins. In this case, the Impulse runs circles around the Alesis, which has pretty much just keys.
5. need polyphonic control? Some novation axes are mono, as are some Alesis but the Q61 is definitely not mono.