Spreads do vary, but which company is cheapest depends on the particular market you want to take a position on.
The difference in spreads is more significant if you are trading actively, e.g., intra-day betting on indices and share prices, where spreads could form a significant proportion of gross gains. In this situation, it might be worthwhile open several accounts with different companies and for each bet choosing the one with the cheapest spread.
If you are taking longer term positions, which in the context of spread betting I would personally define as a month or longer, the cost of the spread becomes less significant and it probably wouldn't be worth the hassle of opening multiple accounts. This is how I spread bet, and personally I use IG Index.