The first question is what forks will fit your bike and your ride. More travel and more control is always tempting for progressive riders but you can't just plug in extra inches of suspension without thinking.
First off check the maximum fork length your frame warranty covers, because too long a fork can easily lever the head tube off if landed hard.
Assuming you're covered, going up 10mm in travel from your existing suspension unit won't affect handling too drastically. 20mm will definitely create a much slacker, potentially slower turning bike though. This potentially means you'll need to add a short stem and wider bars to rebalance the handling and also a forward saddle shift to counter the slack head angle. If you want more travel for descending without disturbing your climbing geometry, a travel adjustable fork in the right stroke range makes a lot of sense.
Plug and Play
Next check the physical fit. We'd thoroughly recommend you use a tapered steer if your bike is compatible. Some 4mm straight gauge head tubes can also be converted to take a tapered fork if you change the lower bearing cup.
While upgrading to a through-axle fork adds security and accuracy you'll need a compatible front wheel, which can add considerably to the cost of upgrading, unless your current wheel can be converted.
Forking Out
Now you've worked out your travel and structural fit limits, it's a case of how much can you afford. There's definitely a progression in standards of control and consistency up to around $630, but after that the waters get a lot murkier, and that's when it's time to be honest about you and your riding.
Best fit
If you're likely to plug the fork in and do the minimum set-up tweaking and then ride it day in day out without servicing it then you want a simple but totally reliable unit. If you clean and care as much as you ride, then you can get something a bit needier. If you're a real fork fettler who'll spend hours with a shock pump and a safe cracker's level of dial turning dexterity to find your suspension sweet spot then it's worth you having a full range of adjustments to exploit. There's no use having lockout and other climb enhancing features if you're an uplift-reliant downhill loon though.
The same ride criteria can also be applied to the weight, stiffness and sensitivity of your fork. Light, tight forks will suit climbers and other XC riders, while superb plush traction Hoovers are worth the extra weight for progressive envelope pushers.