IMO, a handheld VHF is simultaneously an essential item and not much use as the only comms on a boat.
Basically, it comes down to range - a handheld is only 6w max and has a p1ss poor antenna and height above water is negligible - or non-existent if you take a dip. A fixed set is 25w and has a somewhat elevated antenna. The real world transmission ranges are worlds apart. I work with another boat that prefers to use a handheld (don't ask) and their comms are dreadful. Even with my "big ears" antennae, I lose their TX at around a mile - maybe 1.5m at best.
That said - I wear a handheld. In case I fall in and hopefully I'll be heard, or be able to hail a nearby craft. I don't have much faith in that happening in these waters. Handhelds have lots of uses though - especially ship to shore etc. Handy if transhipping PAX too.
I think you're using the wrong eventualities to make your decision. My personal flow chart is:
1. Stay in the boat (if you exit boat go to 5)
2. Use fixed VHF.
3. If fixed VHF fails stay in boat - use mobile phone/hand held VHF
4. If mobile phone/hand held VHF fails, use flares and/or PLB
If you have exited boat suddenly:
5. Use PLB and use Handheld VHF next. Tie PLB to self with antenna clear.
Anyway - I know you have a handheld - that's great, but get that Standard Horizon ordered before the sterling rises