Material -The stronger (and heavier!) the material, the more effective it will be at probing. The lighter the probe, the more expensive it gets. Carbon is the lightest (and most expensive), Aluminum is stronger and cheaper, but heavier, Steel is heavy and expensive, but the best at the job.
Length -You'll see probes ranging from 200cm-320cm. We don't sell anything less than 240. Successful extractions have been done up to 3m. Keep in mind you'll need to hold on to the probe as you enter the snow. Longer probes mean more ferrules, so count on them being a bit heavier (but not much) and having more mass.
Assembly -How quick can you get it together and secured (like really secured!) into place? Practice will help, but make sure it's got an easy lock.