The phones are selling for $724.99 and $824.99, respectively, which is the same price that Samsung is charging. However, unless you pre-ordered with Samsung, Best Buy is your best bet for finding these unlocked phones in stock.
Alternatively, you can purchase the phones from carriers, but that will tie you down to either a monthly or 2-year commitment. If you buy an unlocked version of either phone, you have the freedom of moving to the carrier of your choice without any penalties. You also get a clean, bloatware-free version of Samsung's latest flagships.
The only downside is that the unlocked versions traditionally aren't first in line when it comes to software updates from Google. However, this could be a good thing if you like to avoid any bugs that may come with newly released software updates.
It's worth nothing that if you want a new phone with Samsung's new Gear 360 camera, Samsung is offering its new cam for $49 (normally $229.99) when you buy your Galaxy S8 or S8+ directly from them. The new camera features 4K 360-degree recording and live broadcasting. Just keep in mind you may have to wait longer for your phone to arrive.