Having trained with several types of kick shields (Century, ASP, others) I believe Revgear make the best on the market.
As for the kick shield itself, it is indispensible for high impact, lower accuracy strikes (knees, mule/rear kicks, front thrust/defensive stomp kicks, etc.) or if you have concerns about the accuracy of a given student with any strikes. It works well for both the attacker and the bag holder. The tombstone is awesome for multiple angle strikes and higher precision stuff. Personally, I consider both essential to cover all training, but if I only had money for one, I'd go with the shield over the tombstone, just because in many ways it offers more versatility.
One last thing, a great deal of bag holder \"comfort\" comes from holding technique - something difficult to communicate via written directions.