You have a ton of options, as you have seen, with outdoor speakers.
What you want to consider.
1. Aesthetics - do you want a speaker mounted on the side of your home?
2. Noise polution - a speaker on the side of the house points the audio at your neighbors, would it be better to put a speaker on the perimeter pointed towards the house.
3. Speaker type - do you want a speaker like the wall mount, or an in-ground speaker to put next to bushes, or a rock speaker?
4. Outside speakers & noise - There is a significant lack of bass with outdoor speakers, and bass response is terrible outdoors. Larger (8") speakers or a dedicated subwoofer speaker is really a must for decent audio within any space at all - the larger the space, the more speakers you will need for good quality. Decent quality can be had using less.
Personally I like the concept of using several omindirectional speakers which are buried in the ground to give even converage over a wider area using 8" two-way drivers. You get good audio, don't have to turn it up as much so you don't disturb the neighbors as easily, and they look good.
Rock speakers are also really nice for similar reasons, but they are always out there, visible.
On-house speakers are the 'cheap' ones typically. There are more expensive models like the Def-Techs linked above, but they tend to be more directional and point a lot of audio into the surround home's backyards. Lot's of personal opinons on this though. I have a customer with over 40 outdoor speakers - he wishes he had actually gotten more.