As I grew up playing basketball (junior high and high school), we were always taught to force baseline. In college, our coach taught us to force to the middle. So, as a player, I have had to learn to do both. Now that I am coaching basketball, I force baseline.
Two predominant reasons for this...
1) On the offensive side, I teach my point guards and others to penetrate middle. I want them to get to the lane. If you can get into the lane, especially with a strong jump stop, you are pretty much unstoppable. With a strong, powerful, and solid jump stop in the lane, you give yourself the option to shoot quickly, ball fake and shoot or pass to the low post, or kick out for a '3-pointer'.
2) On the defensive side, I force baseline because I believe that the baseline is an excellent defender. The baseline will not move and the baseline will always get in the way of the offensive dribbler. The backboard is also a good defender in that the dribbler will often times find himself behind it. With the on-ball defender, the off-ball help defender, the baseline, and the backboard, that can make one heckuva trap if the ball-handler picks up his dribble.
One reason I hear for forcing middle is that you are forcing the offense to your help. However, I find that by forcing middle you really aren't forcing them to your help. You may be forcing the dribbler to other defenders, but are really helping your defense out in the best possible way? Where does the help come from?
If the help comes from the low post, nearly any offensive player will be able to easily dump the pass to his low post teammate for a layup. If the help comes from the wing, a good offensive player will kick it out for a '3'. In essence, a nearby offensive teammate will always be open.
By forcing baseline, the low post ballside can help stop penetration. The weakside low post can then rotate over to cover the offensive player in the ball side post. The top guard can rotate over ballside if need be to cover a possible kick back to the ball side wing. The weak side defensive guard can rotate into the lane to cover a high post flash. If defended and executed properly, the only open offensive player is on the weakside wing. The only pass available to get it over there is askip pass. By the time the skip pass reaches its target, the defender will be able to recover and closeout in plenty of time or possibly even intercept it.
In my opinion, too many bad things happen when you let the offensive team get the lane. Back to point #1 above, this is why I want my players to dominate the lane. If you are strong with the basketball in the lane, the defender and his help will have no idea what you are going to do. I don't want to put my defenders in that position. I don't want to give up that many easy shots.
The one and only time I may decide to force middle is I have a phenomenal shot blocker. Funneling the offense to a shut down shot blocker may be of benefit to forcing bad shots and bad decisions. How often, however, do we have that kind of player?
Forcing baseline, I find, is the most beneficial to being a 'shut-down' defensive team. Teaching and executing the concepts of forcing baseline takes practice and drill after drill to get the players to learn it the right way. The thing to remember and understand is that by forcing baseline you are not giving the player a lane to the basketball hoop. I still don't want to get beat on the drive, but if I do, I want the drive to go to my help; not to the middle where his offensive options are endless.