If you were propelling a Space ship with anti-matter, would an antiHydrogen atom or an antiUranium atom be bes

Hydrogen

It is true that a pound of antimatter would provide the same power yield no matter what its composition is but there are advantages to using hydrogen.

First hydrogen is the most common element in the universe so it is easy to refuel, you can skim hydrogen off a gas giant, and you can't do that with uranium.

Second antimatter is not naturally found, we make it using cyclotrons and it is easier to create a smaller element than a huge one like uranium.

Third hydrogen can be used in a fusion reactor and fusion reactors are safer and more powerful than fission reactors (once we get all the bugs worked out).

The fictional design for Star Trek's USS Enterprise uses anti-hydrogen in its warp reactor. The anti-hydrogen is stored in tanks on board (created in space stations) and it is mixed with hydrogen. The hydrogen can be skimmed from gas giant planets or in the case of the Enterprise collected with the Bussard Collectors (those domes at the front of the warp nanacels).

A pound of antimatter is a pound of antimatter regardless. Same energy yield.

I suggest anti-iron; you could suspend pellets magnetically so they don't explode prematurely by hitting the container walls. However, antihydrogen is the only one that can be produced with current technology.