There are many people who know more about the intracacies of board design and materials than I do so this blog will focus on the basics!
Ok so the best way to approach this is to think of a SUP like a boat - the bigger volume of the board, the more water it displaces so to put it very simply big boards are for big riders. If you look at the technical spec of a board (which I really recommend you do before buying) it will give you the maximum rider weight so you can get an idea of which board would be best for your weight. I suggest you keep a bit of headroom below the upper limit just to make sure riding the board in all conditions will still be fun.
Choosing a board too small will mean it struggles to support your weight in the water. A board which is too big will be hard to paddle and you may struggle with paddling into the wind.
For SUPyoga I recommend a board which is roughly 10'0 to 11'0 long at least 30"0 wide. This should be fun for everyone but of course you may need to go bigger depending on your weight. Lighter yogis could also get away with a smaller board but if you get a chance try before you buy!
To put your minds at rest this need not be too exact; I've taught SUPyoga on anything from Surfboards to 12'0 28" race boards, its all possible! And yes you can still do headstands on 8'5 surf SUPs - its a bit more challenging but thats where the fun is!
A quick google search will show you that there are a wide variety of SUPs out there and all with a different purpose! Long pointed boards for cruising and racing, short tapered boards for surfing and everything in between. You need to be honest with yourself and think what you want to use the board for. It could be cruising on a sunny flat day, surfing, river paddles - or you may want to do everything!
A long and pointed cruising board may be great fun on flat calm water, but too tricky in chop, or pushed around by a side wind. An all rounder will be great for having fun in all conditions but may not keep up with your friends if they all have race boards. A surf SUP will make you look like you belong in Point Break riding waves but refuse to stay above the surface on a flat water cruise unless you are paddling like the clappers.
If you can't afford a quiver of SUPs with one for every occasion (which is a considerable investment!) then look for an all round board which will let you have fun whatever you are doing. I'd choose a board which will allow you to progess your abilities and still enjoy it.
For SUPyoga a stable board with a round nose and tail and a large deck pad is ideal so your hands and feet are secure during poses and its soft for prone and supine postures.
3/ Hard or Inflatable
This will come down to your storage, SUPs grow when you bring them into your home and what looks compact on the beach will dominate your spare room and freak out your cat. Most of the people asking me about boards are looking for inflatables as they are just easier to store.
Of course the ease of storage is then weighted against inflating your SUP everytime you want to use it which will certainly warm you up for paddling! You may also have a smug group of friends with hard boards drinking their coffee and commentaing on your progress, but this only takes 10 mins or so and then you are away!
I have had very good experience buying my boards from Red Paddle - a company who specialise in just making inflatable SUPs, they do nothing else and are experts in the field. When considering an inflatable SUP you want to make sure that it is able to inflate to a high pressure - the higher the better - so that it feels more like a hard board. Nobody wants to go paddling on a lilo! Never exceed the maximum inflation pressure - you dont want to burst your board or end up with a hernia.
I prefer inflatable SUPs for SUPyoga as the deck pads are softer and the boards seem a little more sympathetic.
Now I'll be the first to admit that I have expensive tastes; my piggy bank with "Naish Alana 9'5" written on the side is testament to the theory that if price equals taste then my taste in SUPs is simply magnificent. My advice will always be to buy the best you can afford. If you are buying for you and your partner then perhaps consider different styles. I bought an all purpose SUP for my husband which we can both ride comfortably, it does surf and is also good for yoga. For myself I bought and a longboard style surfer SUP which I can ride comfortably everywhere and we can both share to go surfing on wavey days as it does surf better than the larger board. A small compromise but the best I could afford and we both get to have fun in all conditions.
I would be very wary of going for a cheap inflatable SUP unless you have tried a variety of boards and are happy you will be able to enjoy it in a variety of conditions. Sadly you do get what you pay for so if like me you want to use your boards as often as possibe without worrying about them then focus on buying quality. Read the reveiws and speak to shops and suppliers, the last thing you want is to spend hundreds of pounds only to be disappointed a few months later.
Choose a board from a well know brand, with a high inflation pressure and try and ride before you buy. Schools will often sell off their boards at the end of the season so there are bargains out there to be had! Just make sure second hand boards are as described - don't expect them to be in mint condition but I wouldn't accept too much damage as you will have to pay to fix this. Having to replace handles, fins or deck pads is annoying as its time off the water and not what you budgeted for.
If you buy a package (board, paddle, leash, pump, bag etc) it is money well spent to upgrade the paddle. I started out with two alloy paddles that came as standard in the package and replaced them both with carbon ones within 6 months. An important lesson in buying kit you can grow into!
Aloha and happy paddling