If you are position trading (ie holding overnight or for several days) then using a SB company makes more sense. If you are trading intra-day then in my view you really should use a direct access broker.
I had a look at trading small companies with a spreadbet firm. Problem is, due to the low volume in those sort of markets it is difficult to get a reasonable quote from a spreadbetting firm. Also for a share that cost something like 8p a share they would only allow me to place my stop at 0.01p a share.
I use to trade a lot of AIM listed stocks and found it far less hassle to just use a broker. I would hold position from 2 hours to about a week. I never made more than 1 trade in a day and sometimes only made 2 trades in a month.
Unless your trading companies in the FTSE 350 or similiar sized companies in non U K markets forget Spread Betting companies there isnt enough volume for them to make a decent market and therefore you will have problems.
If you are shorting I wouldn't use spreadbetters but prefer CFDs as you will not only get a better price but you will get interest on the trade whilst it is open. But you will have to pay CGT.
I use spreadbetter for longs on large caps if for a long period of time.. e.g. months, as long as the spread isn't too wide. Small caps, very difficult with spreadbetters as already mentioned