Tin is potentially more recyclable than plastic and while it's even less degradable it's less harmful in the environment when dumped.
Making the tin is more than "how much resource is involced with digging up tin". Mining is resource-intensive and not environmentally friendly, but it's only the start. The ore has to be smelted to extract the tin, which leaves a lot of other stuff to be disposed of. Tin ore is only a few percent tin, so most of what it mined has to be disposed of somehow after using more resources to extract the tin. Then it has to be refined, which uses more resources, and processed, which uses more resources, and shipped, which uses more resources.
More resources will be used in making and distributing tin foil than plastic wrap, but tin will have much less of an environmental impact after being dumped.
To some extent it's a moot point because tin foil simply can't be produced in adequate quantities to replace plastic wrap/bags.
Flavour shouldn't be affected by hours of contact with either material. The amount of leaching would depend on the filling of the sandwiches, but I doubt if it would be significant with either. A comparison would be difficult anyway because you don't specify which plastic. There are many plastics.
I'll throw in another option - a sealable box that you wash and re-use. That would be less harmful to the environment than single-use disposable wrapping.
Or you could go really old school and use a cloth wrap. Linen was traditionally used for wrapping food in this part of the world (with muslin gaining popularity later on) but there might be something more effective and/or environmentally friendly nowadays.
