Just curious since both version come with the retail purchase. And are there any drawbacks to using the 32 bit Vista in comparison to using the 32 bit XP Pro?
Both verions coming together would be rather rare. Where did you see that offer? As a rule each version of Windows 32 or 64bit is packaged and sold separately. As far as Vista having compatibility problems you can expect some automatically since that has been seen with every version of Windows since dos.
Everyone so far has found XP runs faster then Vista. Of course everyone was trying out the beta versions until now. Most planning to eventually upgrade are waiting until some service packs are later released to correct any initial goofups seen in the first release. The 32bit version should be quite a bit easier to use as a rule since at the present time there is very limited software support and products for the 64bit versions of anything.