If you are going to have to pay full retail price for a cell phone you might as well buy it unlocked if the option is available. According to a Best Buy listing they plan to offer unlocked variants of the phone beginning May 9th. The phone will certainly work on GSM carriers, but I have not read anywhere whether or not it will include CDMA radios. The unlocked variants will also come with a $25 discount being priced at $724.99 for the S8 and $824.99 for the S8 Plus in Midnight Black on either variant. Unlocked phones are great because they don't include all the bloatware that their carrier version counterparts carry. The phone actually feels like it belongs to you as oppose to being rented by the carrier.
The only drawback may be the speed at which Samsung has been updating their unlocked variants. The unlocked S7 and S7 Edge have been the slowest to receive updates so far. The listing links can be found below.
Unlocked Galaxy S8
Unlocked Galaxy S8+