I have operated accounts at both SBO and 188. Both do offer competitive AH odds for football (soccer). For other sports they are not so competitive. However there is a big difference in the size of bets they will accept with SBO way out in front.
Further, based on my experience, 188 will reduce your bet limits if you show any sign of knowing what you are on about. My limits were halved, halved again and finally got so low that it was no longer worthwhile maintaining an account there. Based on reports I've read elsewhere that is common practice at 188, just as it is with all the Euro bookies. On the other hand, I've not read any reports of SBO limiting or banning players.
My advice would be to open accounts with both and avail yourself of the best odds you can get but be prepared to be shut out at 188 in due course. I've found both to be prompt with payouts (using Moneybookers) and no hassles dealing with either.