At the end of it all, if you are betting with a bookmaker that doesn't offer best odds guaranteed, the SP is likely the better option. Of course, this ultimately comes down to how confident you are in that horse and how much research you have done on that race, as the odds could change come to the race itself.
With standard UK and Irish horse racing when you choose SP then it doesn't matter what the price is when you take the horse, it only matters what price your horse is when the race starts - this is what you will be paid out at.