Impedance or sensitivity?

Depends on the amp. Please tell us if the amp you want will be tube or SS.
Tube amps usually don't like low impedance speakers especially OTL amps. Impedance is hard, if not impossible, for tube amps to deal with when it gets lower. Tube amps can deal with low sensitivity by offering more power. It is costly, but not a limitation in many cases.

So in general impedance is harder for an amp to deal with. Solid state amps don't like high impedance. For example Soundlab speakers will have bass impedance in the 40's causing trouble and possible osillation for SS amps. They put out far fewer watts into high impedance loads - a small fraction of their rating.

It's worth considering too that the impedance spec is nominal. Impedance can dip below it and rise above it at various frquencies, and can have varying combinations of resistance, capacitance, and inductance. So two speakers with the same impedance spec can present an easy and difficult load respectively.

josh358 is right. Just because the stated speaker impedance is listed at 4 ohms does not make it harder to drive then all 8 ohm speakers. The important thing to know is how much and widely the impedance varies. A 4 ohm speaker that never dips below 4 ohms may well be easier to drive then an 8 ohm speaker that dips down to 2 ohms in the highs. Great point!