The 2 piece is the factory OEM design. You can replace just the outer "half", as this piece has the ball joint in it. The only alignment adjustment that could be affected by this is your caster, as the purpose of the 2 piece design was to allow easy caster adjustment of the passngr side, which in turn allows for adjustment of caster split.
If you are very careful in measuring the postion/number of thread turns on your old bolt, you could probably change the outer piece without needing an alignment.
On the other hand, switching to the aftermarket one-piece design will require removing your entire right upper control arm. In this case, you will need an alignment immediately after installing the part. You would also lose some of the adjustablilty that the front end had from the factory. It may or may not prevent the alignment shop from getting it "perfect". However, what you gain is new control mount bushings, and most of the aftermarket 1-piece control arms have replaceable ball joints, so that in the future you can just replace the ball joint.
