Being a lefty, I have a different view on ambi selectors. I've got one on one of my ARs, and frankly, I don't like it. When I use the ambi selector, the left side of it hits the top of my hand by the first joint. Sometimes that prevents the selector from moving, sometimes it just slows it down. I've thought about doing some WECSOG on it and lowering the profile to help alleviate that.
Here is another thing to consider. If you have an ambi selector on your AR, or even a lefty only model, and you train (hard) with it, you will learn to use it that way. That is great, until you pick up another rifle in the heat of battle (yours is damaged, destroyed, hopelessly jammed) that doesn't have that particular selector. Now your muscle memory and training are fighting against you, trying to use a selector that isn't there! Bad juju in my book. For us SHTF-minded civvies, probably not as big an issue (we aren't likely to have another AR to pick up) but for guys in the sandbox, it could be fatal.
I'm of the opinion that basic controls on an AR should be left standard. The AR is actually fairly lefty friendly as it is. I use my trigger finger to change the selector and actuate the bolt release. I have installed a Norgon ambi-catch, but frankly, it isn't quite right (not enough leverage.) So I use my right thumb to drop the mags. Train to use the weapon in its standard configuration, and you will always be able to grab any AR and use it right away.
I think the worst possible idea is the Stag Arms lefty upper. Talk about really screwing you up...
YMMV.
Scott
ETA: Suits heart is in the right place though!