How about your sightlines? Lower down position, does that mean you can't see as much? Is it noticeable over an upright?
I don't commute, but there's plenty of traffic between here and the nice lanes and/or the railway station, so I feel qualified to answer this:
HPVelotechnik Streetmachine: Same eye position as when driving a car. Similar 'bonnet' length to a small car, too. So not quite as good for seeing as an upright bike, but perfectly okay. Getting a foot down is tedious in start-stop traffic. Filtering is possible, given sufficient room, but there's rarely sufficient room (it's not so much the width of the bars, as the wobble room you need for the boom if you stop and start). Definitely gets more respect from drivers than an upright.
Optima Baron: Seriously low. You can't see past cars (effectively every car becomes a transit-style van), and the bonnet length problem at junctions is exacerbated. A rigid lowracer with road bike tyres is extremely unforgiving of road surface hazards, which means a large amount of your observation and planning goes into avoiding them, and you sometimes have to manoeuvre round things that drivers aren't necessarily expecting you to. Unfamiliar roads become hard mental work. On the other hand, being fast means you can keep up with moving traffic better. Getting a foot down is less physical work than the Streetmachine, but starting and stopping requires non-trivial skill, and there's always the possibility that you'll drop it when starting off on a dodgy camber. Filtering is deeply unwise, and passing right-turning vehicles on the inside should be done with caution. Looking fast means drivers respect you, but you don't have an awful lot of profile end-on, so they may fail to spot you or misunderestimate your speed. Drivers are typically very cautious when overtaking, as you disappear behind their nearside wing.
ICE Sprint: Slightly higher eye position than the Baron, but you still can't see past cars. Three wheel-tracks means you need to pay a bit more attention to the road surface, but the failure mode if you get it wrong and hit something tends to be no worse than a puncture. Similarly, you won't fall off if you lose control on ice/diesel. You never have to put a foot down to balance, so start-stop traffic is boring rather than work. You can forget about filtering. Drivers tend to pass you as they would a car, because you *look* really wide, even if your actual width isn't all that more than a set of modern MTB bars. Flip side of that is more abuse from those unwilling to overtake as you winch yourself uphill.
Being able to see less is something that will affect how you ride, but I don't think it has much impact on safety in normal circumstances, because you're in control when pulling out at junctions etc and can compensate accordingly. You may end up stopping and waiting more than you would on an upright where you can just see that a junction is clear on your approach.
One problem with a low riding position is that it puts your eyes in the dipped beam of car headlights. This is about as fun as it sounds. The other problem is that the lower you are (unless it's in a velomobile), the more of you gets splattered with spray by the tyres of passing vehicles, and if you're particularly unlucky you may get your face gritted in winter.
As Mr Larrington says, at night recumbents tend to lose some of their WTF factor, at least when viewed from the front. Trikes may do a passable impression of a distant car when viewed from behind, depending on lighting arrangements.
If I were commuting, and not compelled to three wheels for disability reasons, I'd want the highest riding position that I could easily get a foot down from. USS or open-cockpit bars are probably a good idea, as you can sit forward in the seat to peer round corners. And preferably something that's not ridiculously heavy if there's going to be a lot of start-stop involved.