I don't know about Reliv but I compared Ensure and Boost and I thought Boost was much more nutritionally complete than Ensure. It comes in Boost and in Boost Plus which has more calories if you are depending on it for your main nutritional source.
The bottom line, I think, is to compare taste because you are the only one who can make that decision...no matter how good a product is, it won't do you any good if you don't drink it because it tastes nasty.
I also found it easier to put my boost in a blender (I used one of those "Magic Bullets") with ice and sometimes a piece of fruit and/or a little milk. It tasted better and made me more likely to drink it.
Good luck!