The AngleGrid concept and the Isogrid concept are very similar. The only difference is that the AngleGrid lets you specify the angle variable on the Variables tab. If you choose only Isogrid, then the software will ignore the Angle Variable on the Variables tab and will use an angle of 60 degrees. Assuming the ribs are all the same height and thickness, this means that the panel will have the same strength in the 0 and 90 directions, hence the name "Iso" grid.
I hope this helps.
*UM Basic 6.1