The more important question would be, "What truck are you going to tow with?"...Also, what is the GVW of the two 5er's you're interested in...Don't go by "empty" or "dry weight". You need to go by the actual GVW of the 5er and take 20% of that to "ball park" the amount of pin weight you would be putting in the bed of your truck...
But more information is needed: What truck and model and GVW's of the two 5er's you're looking at....
thanks
On Edit: My 5er has a total length of 39' 9"...I tow with a 3500 crew cab/long bed/ dually, no problems...