Either one can cause issues with enough vibration. Spade can work loose, Solder onto the speaker tab is strong there but where the solder ends on the wire can be a weak point. I'd use spades and add just enough of a crimp to them so make it more difficult to work loose. Also, spades are easier to put back on if they come off compared to a wire breaking that was soldered on.
Buy the good connectors. The ones the have the heat shrink built onto them. They're great for using them in places where the elements can get to them but they also offer a more secure connection with the heat shrink holding the wire
+1 for spade connectors. I think I have 3 sets of crimpers now. 1 crappy set for crimping regular terminals, 1 ratcheting set for regular terminals, and 1 set to crimp deutsch pins. I just installed an aux fuse panel on my ride and bought a 15.00 Harbor Freight heat gun to use for the heat shrink - makes all the difference.
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