Basic SUP terms and other factors to consider
Buoyancy - This is probably the most important factor to consider when selecting a SUP as a beginner to intermediate rider. The length, width and thickness of the board combined with the shape of the board will determine the volume of the board. The volume of a board is commonly converted into Liters. A board that has adequate volume for the riders weight will float higher in the water and will tend to be more stable and paddle easier and faster than a board of less volume that sits lower in the water.
Stability - The combination of a board's volume and width will determine the back and forth stability of the board. When standing square to the board, the back and forth stability (left to right) is what will keep you dry and out of the water while paddling. Generally surf models will be in the 28"-29" range while flat water boards will be in the 30"-32" range. Anything over 30" would be considered very stable. The draw back to a wider board (above 32") is that it will be harder to turn and will be less responsive in the surf.
Length – The overall length of the board will determine the amount of surface area the board will have in contact with the water. Generally speaking the more surface area (board length) the board has the faster and better it will paddle. Like a sail boat, the longer the hull (board) the better it will cut thru the water and chop and the faster it will go. Having said that, you may not want to run out and get the biggest board you can find as you still have to carry it, store it, transport it and control it in and out of the water.
Volume – The volume of the board directly relates to the buoyancy of the board. It is often calculated into Liters (L). See buoyancy for more info.
Performance - Performance can be interpreted many different ways but here are some basics. For flat water, a high performance board would be one built for speed and distance paddling, getting from point A to B, as fast as possible. Typically the higher volume, longer length (12'6" & above), and sleeker designed boards perform best. In the surf, a high performance board would indicate one that is easy to turn and very responsive. This would entail design features (bottom concaves, tails, rails) and fin set ups found in surfboards to be used on SUPs with the least amount of volume as possible for the riders weight. For recreational paddlers and surfers it may not be in your best interest to start out looking at the most performance oriented boards but rather boards that can accelerate the learning curve and perform well in a wide variety of conditions.
Maneuverability – Maneuverability is meant to be a way to measure how easy the board is to turn or change directions. In flat water, this is much less important than in the surf. Flat water paddling is very similar to kayaking in that you paddle on both sides to keep going in the same general direction and to turn you use the same paddling techniques. Also similar to kayaking, in recreational paddling you typically head long distances in a general direction and are not concerned with quickly turning the board around, thus maneuverability is less important. The volume of board will typically determine how easy a board is to turn, the more volume the harder, the less volume the easier. In surfing maneuverability is much more important and will determine how quick and responsive the board will react to the rider turning the board. This again is directly related to the volume of the board and other design features of the shape.