This topic comes up from time to time on the Board, so a search could find another thread that will provide your answer. Regardless, here's some thoughts that might help you out.
BGS is the most respected of the bunch and they also seem to grade the toughest. So expect to pay more for a high grade BGS rookie. For example, my friends that regularly grade their cards to resell them, will try BGS first for a card that looks in great condition. If it doesn't get a 9.5 grade (let's say it grades at 9.0), they'll usually take the card out of the plastic case and submit the card to PSA, because they often get a PSA 10 (highest PSA grade) back from PSA. A PSA grade 10 card will typically sell for more than a BGS 9.0 card, although like anything, there are exceptions.
Keep in mind that a graded card doesn't necessarily mean that you're getting a card that's not been tampered with. I've heard of examples where cards have been repatched (the original patch removed and a better color/design patch substituted) or cards that have been laser cut to remove edge flaws (so the end result is a smaller card) that have been graded high by BGS or other card grading companies. For many, cards that have been altered have "zero" value. Most of the time graded cards should be OK but you must recognize that unscrupulous sellers are out there and a graded card isn't a 100% guarantee of the card being AOK. If you're buying a really high end and expensive graded RC, you may think about dealing with a reputable seller and asking him about the card history before cementing the deal.
Good luck with your search!