The double blood donation obviously takes twice as many red blood cells out as a single blood donation. The difference is that in the double blood donation the plasma is put back into you and also saline is added so that your blood volume doesn't drop.
From the standpoint of "feeling" after the donation the single or whole blood donation is more likely to cause a drop in blood pressure and consequently some dizziness because of a lower blood volume, all which doesn't occur from the double blood donation. From a oxygen carrying capacity the double blood donation would have more of an effect.
If you or your doctor is concerned about a clot or stroke due to high hematocrit he should write you a prescription for a therapeutic phlebotomy. Every sixteen weeks for a double red cell donation and every eight weeks for a single donation is not enough to consistently keep your hematocrit at the level you would like. It is replaced very quickly by the body.
Do watch out for becoming anemic. When you donate blood and they check your hematocrit level before you donate, if it is not 46 don't donate.