Check the commission structure. If you are only going to be trading small size then the minimum commission probably means that the SB option is much cheaper. Conversely, if you are trading large size then CFD will be cheaper.
check out the platform, via a demo.
check their spreads against the underlying market
check their spreads against competitors
check their spreads after hours
check the minimum lot sizes
functionality of their charts if you will be using them
their order options, targets, trailing stops etc
make sure they have all the products you might want to trade
look at positive/negative reviews
minimum account opening and charges
Money talks, so if you are not happy, feel free to shop around
Further to barjon's post above, this FAQ might help you to compare and contrast the two: What are the Pros and Cons of Spread Betting Vs CFDs? One thing to keep in mind before getting into too much detail is whether or not you can trade a spread betting account from your part of the world (just looking at your flag). Furthermore, even if the SB brokers are happy for you to open an account, you may fall foul of your own tax authorities. A couple of e-mails / phone calls to the respective organisations will soon sort that out.
It's really difficult to choose the right broker? As I'm now based in UK, dealt with different UK brokers. I always avoid Cyprus based brokers. As I found out most of them are not using real time platform and also they provide most facilities than any other cuz they know at the end you'll get nothing. Most of the UK's brokers are FCA (Home - Financial Conduct Authority) regulated so, you can take action any time doesn't matter even you're from abroad which is very useful I think. You can try following brokers. Please make sure you contact all of them. As you're new in trading it's easy to lose your capital.
I would say the main things that you need to check are whether the brokerage offers advice on trades or is an execution only company, plus I would check that they are regulated by the FCA. If the brokerage offers execution only then all you are going to get from them is, research ideas, if they offer that service, and information by either email or phone that will help you make your mind up on trades etc. If you go for an advisory brokerage then you will obviously get more for your money, the brokers will advise you on what to do as a result of certain stock movements, it really is down to preference though. One final thing I would check is charges, how much commission do they charge? This is vital information to obtain before you place any trades, the last thing you want to do is place a trade then find out they are ripping you off with commission!