Probably not much right now, depending on where you are. Band 2 is used mostly in rural areas, where T-Mobile doesn't own enough 1700/2100 spectrum to put LTE on Band 4.

Band 12 is the low band spectrum T-Mobile is using to give better in building penetration in cities, though it will also give better coverage in rural and suburban areas as it is built out. Few phones have band 12 at this point, mostly just phones released in the last 6 months to 1 year, but it will be quite useful to have. The biggest issue is that T-Mobile hasn't rolled out a lot of band 12 yet, and they don't own the spectrum in all areas, but they are trying to get it rolled out quickly.

So, for the easy answer, it likely won't impact you much now but having them will significantly improve coverage, especially in another year.