Press release
Storefront, including list pricing, specs and box contents. Note that the devices lack FCC approval and cannot be ordered yet. You can sign up to be notified by e-mail when the devices become available for sale.
Based on the information above, it appears that the SE+ is more or less functionally equivalent to the original Explorer. That is, it has stand-alone navigation capabilities (without being paired with a smart phone using the Earthmate app) but no on-board maps. The Explorer+ appears to be functionally similar to the original Explorer but with the addition of on-board maps.
There are a lot of questions on social media regarding the extent and quality of the maps. So far, the only answer is that the maps are "DeLorme topo maps". Historically, DeLorme mapping is quite good for North America. Quality and resolution varies elsewhere in the world.