I'm thinking about switching to Reserves with 16 years all Guard. What is the advantage for Officers in Reserve vs Guard for retirement?

There is no advantage. Both are based on the same points system. I've been in both the Guard and the Reserves but my Guard time was as an Enlisted Soldier. I didn't look at your profile before posting this so I do not know what your branch. The Guard is more combat arms with some combat support and combat service support units. The Reserves is more combat support and combat service support with one combat arms unit based in Hawaii, Guam and American Samoa. If your branch is combat arms unless it is aviation then I would suggest that you stay Guard but if it is combat support or combat service support then switching to Reserve would be ok.

CPT (Join to see), I've been retired for some time, but I served in the Alabama Army National Guard but did most of my time in service and finished my career in the USAR. Everything that MAJ Anthony Henderson stated is correct.
However, there are a few more items that you might wish to consider as well. The most important difference to me was promotions. In the USAR, when I got promoted, I was able to accept my promotion. You may have to find another unit, or even go to the IRR, but you still take your promotion. In the guard, they determine if you're to be promoted or not, even though you've passed a Federal board. I know many officers that have left the guard for the USAR in order that they might accept their promotion.
Also, if your civilian occupation subjects you to relocation, transferring from one unit to another within the USAR is much easier than transferring from one state's guard to another. I went from captain to lieutenant colonel, living in Cincinnati, New Orleans, Oklahoma City and Nashville and never missed a drill.

Email me sir.. MSG Woods .. [login to see]