Z-wave versus Zigbee
The SmartThings hub is one plastic case that contains two separate network controllers, one for Zwave Plus and one for Zigbee (using the Zigbee Home Automation profile). So you can have both zigbee and zwave devices connected to your hub.
When it comes to selecting an individual device like a motion sensor or a doorlock, there are pros and cons for both.
Zwave: the fact that boosted Wi-Fi does not interfere with Zwave sometimes makes DIY installs easier for fixed point devices like locks, window coverings, and light switches. (The things you can't just move 2 feet to the left.) Historically, I believe this is the reason for the dominance of zwave for DIY devices in these categories in the US. However, as Tim mentioned, the "beaming" repeater requirement sometimes means you have to buy an extra repeater just for the zwave lock. Zwave is limited to 232 devices per network and 4 "hops" per message. And it basically lives in its own world, which is good and bad.
Zigbee: usually better battery life and smaller devices. Also tends to travel a little better through water (both rain and pipes in the wall). Depending on how they're configured, Zigbee networks can handle thousands of devices, which becomes important for hotels, hospitals and big apartment buildings.
The zigbee home automation profile which SmartThings uses allows for 15 hops into the hub and 15 hops out for a total of 30 hops for one message. That can be particularly useful in a house that has architectural barriers like brick walls, foil backed insulation, or chicken wire plaster lath, because you may need extra hops to get around corners.
Because power management is a little better than Z wave, a number of community members report that the inexpensive zigbee sensors are more responsive than the inexpensive Z wave sensors, probably because they can be set to report more often without worrying about using up the batteries too quickly.
Having only one standard internationally, as Zigbee does, is good, but in practice different manufacturers add their own encryption layers anyway, so it's all too easy to get a zigbee device that doesn't work with another Zigbee device. Different countries also have different power output regulations, which means a lot of the "amplified zigbee" devices which are used in the US cannot be sold or operated in the EU, for example.
Professional installers from companies like control4 tend to like zigbee because they already have the tools to deal with proper device placement and mitigate interference, and they like the proprietary stuff that you can do with it because they can create brand differentiation.
Just as an example, the built in secondary controller logic that is in Z wave is usually pretty easy for DIY people to use (although it's not supported in the SmartThings multi protocol concept). But control4 built their own secondary controller architecture over Zigbee, and it's great. (Super expensive, but great.)
As far as the future goes, Zigbee and thread (from Google) can run on the same physical devices. Literally the same device can run both protocols. No one's quite sure how that's all going to play out yet, but The thread group and the Zigbee alliance do have a partnership, and Zwave is kind of out there on its own.
So as always, preferences come down to individual needs. The DIY landlord who has a duplex house with two apartments that she wants to outfit in the US would probably find zwave easier to deploy.
A professional installer Setting up in a 2,000 bed hospital will almost certainly prefer zigbee.
Outside of that, it's just going to come down to a lot of individual factors, and many people prefer a mixed protocol option like SmartThings offers.
SmartThings participates as a voting level member in the standards bodies for both Zigbee and Zwave.
Zigbee Alliance official site 
Zwave Alliance official site