The F-105W is the F-91W perfected.

I have both and I thought so too at first, but compared to the F-91 the F-105 has significantly lower contrast display (darker background) and is harder to read in daylight. The F-91 has slightly larger numerals too.

I've come to the conclusion that the F-91 is better for urban conditions where there's always light and/or a cell phone available, but the F-105 is probably better for going off-grid.

(some context for bystanders: the F-105 is the "illuminator" version of the F-91, so the light feature works much better.)