Without contesting any of the above there's an interesting slant to the question of FL or Alt. As long as I know the terrain clearance it doesn't matter whether the sub scale is set to 950hPa or 1050hPa.
For instance if I take off from the top of a mountain (highest elevation in 100nm) and the sub scale is set to 950hPa with the altimeter reading +800ft then as long as the altimeter reads +1300ft I'm 500ft above the highest obstacle within a 100nm radius.
That said if I were to go sightseeing locally within perhaps 1-2 ASRs, flying an altitude is probably more sensible than a FL as only one or two sub scale setting changes are required. But flying through 4-5 ASRs or more sometimes in quick succession would mean 4-5 (or more) pressure changes followed by the necessary climb/descent adjustment when a FL altimeter indication would remain the same (notwithstanding terrain clearance minima).
In sum: local sightseeing its more useful to fly an altitude affording liberal descent/climb manoeuvrability, for long legs crossing several ASRs use a FL for simplicity. Simples!
GW