For the original question.
All the in production dual band antennas are going to be fairly comparable, so pick the one that you can afford. The more gain it may have (within reason) the better off you are. I'll add the Comet line of dual band antennas in the rest of the opinions already expressed. Are they 'better' than all the rest? I haven't the slightest clue, but the ones I have are certainly comparable.

As for a monitoring antenna, since you've got a 50 foot tower, run a wire antenna off of it. Cheap, variable length possibilities, and they tend to work. If it's strictly for monitoring, just run a wire as long as possible and end feed it. Will be good for transmitting? That's not likely without a lot of effort, but it'll 'listen' real well. And it's cheap.
There is no such thing as a 'do it all well' antenna. They are about like all the other "one size fit's all" thingys, no they don't.
And probably the most profound argument you can be given is that it's VERY doubtful if you only have one antenna for anything. Or one antenna for very long...
- 'Doc