I have the soy mirrors. Get one for each side or you’ll be fairly blind to stuff on your left hand side like upwong riders trying to undertake on a club ride.
My take on this is that a right mirror (or left, in
ABROAD, where the
FOREIGNS come from) is all you need for normal road traffic, apart from the rare event of having to merge to the left (eg. when coming off a flyover).
If you're going to be using shared-use paths, track racing or riding with habitual undertakers or similar is when the left mirror becomes invaluable.
I manage fine with one mirror on the tourer (I have a sacrificial bell occupying that space on the left to protect the shifters if it falls over), but fitted two to barakta's trike in anticipation of a lot of shared-use, and quickly fitted a second to the Red Baron when I started doing BHPC races.