Tip #1: Let me make this one easy rule for you:

Do not buy cheap Chinese wireless motion sensor alarm systems!
There are many of these in the market, typically at less than half the cost of other systems.
They come with long lists of features and a poorly translated manual that is good for a nice snorting laugh. Read my lips: You get what you pay for. There is a reason those things are so dirt cheap. If you go for brain surgery, and you know someone will be scratching around inside your head with sharp tools and whatnot: Which surgeon would you choose?
The one that is 2 x cheaper than the rest? Didn't think so... If you don't trust a cheap brain surgeon, why would you choose the cheapest wireless motion sensor alarm system to protect your family?

Tip #2: Start not with what, but who...

For a moment, let's ignore the wireless motion sensor alarm system and its features, and we consider the company and its people. If you are considering buying from an agent or online, find out who the main distributors of the product are. They are the ones you are actually dealing with in the end.
Make sure the company is legit. Phone them and ask a bunch of questions. Send them an email - See if they respond. Can they get the basics right? You surely do not want to drop a tidy sum on a bunch of people who cannot be bothered to send a timely response to an email? Make sure that help is available when you need it. Make sure these are the types of people you would like to do business with. Chances are you would be prepared to pay more for a product whose people gave you that warm fuzzy feeling because they made you feel safe

Tip #3: Look for what others are saying.

Before booking yourself into hospital for that brain surgery, you will probably want to ask around to hear about this doctor's reputation? Is he clumsy? The same goes here.
Ask around! Ask your friends and family, ask your 5400 Facebook friends (who you like to ignore when you run into them at the mall!).
Read the reviews if there are any. When you call the company (from tip #2), ask for referrals, speak to someone who actually purchased a wireless motion sensor alarm system.

Tip #4: Compare the total cost of ownership.

I am fresh out of brain surgery analogies, so let us consider buying a car: Clever people will tell you to ask about the warranty, the service plan, parts availability and service costs. The same goes here. Are there hidden subscription / recurring costs in owning this particular wireless motion sensor alarm system? How much will you pay for the batteries and how often do they need to be replaced? If the outdoor sensors become faded from the sun, can they be refurbished? At what cost? If something breaks, can it be fixed? Where was it made? 

Tip #5: Note that this is the only tip where I will mention functionality, and it is this:

Make sure how exactly the wireless motion sensor alarm system notifies or alerts you when movement is detected? Is it a recorded voice? A siren? A beeping sound? A flashing light on a panel? - This is critically important because if the information about what is going on, is not communicated clearly, it does not help you at all...

There. You can now consider yourself educated on the finer details of buying these systems.
Go forth and be safe!

Full disclosure: We sell the Drongo wireless motion sensor alarm system, and if asked I will obviously tell you that it is the best thing in the market, but selling Drongo stuff is not what this article is about. I am giving you broad advice, and if that leads you to buy a competitor's product, I will be happy that you have done so after receiving excellent advice!