Yeah, I think this is kind of the most critical issue as any of the salts mentioned would probably work fine but it depends on what you are trying to accomplish with your water changes. If you are counting on the water change to "rebalance" your alk/ca, then you may opt for one of the varieties that mixes up to higher levels than your tank set point (e.g. water change mix Alk at 11, tank target 8 or9) but, as monkeyboy suggests, you want to be careful of creating significant swings - which will depend on how much water you are changing out, how frequently, and consumption rate. Alternatively, if you plan to manage your alk/ca levels through some other mechanism (e.g. dosing, reactor) or you have really low consumption (all softies) you would probably pick a salt mix that more closely matches your tank setpoint. Regardless, because of the potential to create swings through water changes in a smaller tank and potential uncertainty around consumption rates in a new system I would recommend actually testing the Alk fairly regularly until you have your process dialed, whatever salt you choose.
Good luck!