Things to consider when buying a paddle board SUP.
If you are planning on surfing waves on your sup buy an epoxy/rigid sup.
For almost all other paddling conditions an inflatable or isup is a great option. isups are more durable, lighter, easier to transport, and when properly inflated, offer comparable performance to epoxy/rigid sups in most conditions. isups also usually come as a complete package with paddle, pump, bag so you are ready to go.
The dimensions of the board, (height, width, thickness), will mostly control the performance characteristics of the sup, along with the fins. It also determines the boards volume, which governs how much weight the board can carry.
Height: will mostly govern the boards trackability, or how straight it paddles. the longer the board the straighter and usually faster it will go.
Width: will determine how stable the board is. Boards 32" and over are considered "stable", boards under 29" are more focused on speed and glide and are less stable. 32" is most common.
Thickness: will contribute to the boards overall volume and add to its carrying capacity. Thicker boards also help to keep your feet drier and away from the water, and warmer in cold water. Inflatable sups usually come 4" or 6" thick. There is not really much of a disadvantage to having a thicker sup.
There are various fin configurations, but all basically achieve the same result on flat water. The longer the center fin, the more stability it offers. Small side fins add a little more stability, and help a little with tracking. If you are paddling in predominantly shallow water, consider a shorter fin to avoid hang ups and breakage.
Slide style:(most common)
Longboard style:(US FIN BOX)
Epoxy/Rigid Longboard US FIN BOX
**Epoxy "longboard" style fins are compatible with most inflatable "longboard" style fin boxes