If you just want one or can only get one light for trail riding, which one depends largely on your primary type of riding, your terrain, and what you already have for lights.
First, helmet and handlebar lights do very different things, which is why we design some of our lights specifically for handlebar use (Trail Evo) and some for helmet (Hangover), then some with the ability to switch between depending on use case (Portal).
Handlebar lights are pointed where your bars are, which is not always where you need to look, which is why we designed the optics for Evo to create a SUPER wide and even beam pattern, so that when you counter-steer into a switchback, the beam doesn't totally lose the trail. Additionally, a light on the bars really helps convey depth on the trail by projecting shadows out from rocks and roots sticking out of the ground, making the terrain look 3-dimensional and much easier to process at speed under you, so we want a good handlebar beam pattern to fill in the entire foreground for you.
Helmet lights always point where you point your head, which means you can use a narrower beam to focus the light more specifically, which means you can use less power and a smaller battery to get comparable runtime as the bar lights, reducing the weight on your head. Helmet lights help fill in down trail at speed, around tight corners, and into drops and holes. Having too much power can wash out all the depth you get from your bar light and make the trail look much flatter than it is, so can pointing a helmet light too far down, so balance is key here.
So which one should you get if you just get one? That depends.
If you already have A light, then chances are it's fairly narrow beam, as most lights on the market are, in which case it's going to be better suited to helmet use, so we would recommend putting that on your helmet, and looking at our Trail Evo to put on your bars. If you're not sure what you have or what it's best for, shoot us an email, and we'll help.
If you have no lights right now and need one to get started, the best choice will largely depend on your terrain and primary type of riding. Out in the desert, for example, everything is wide open with few trees or obstacles to interfere with your light, so Trail Evo by itself is a great single-light solution, particularly since terrain can be quite rough, and a bar light will help you read the terrain much more accurately than a helmet light by itself. On the flip side, in the Pacific Northwest, trails are steeper with lots more foliage and tight corners, often smoother terrain, so a helmet light by itself can be great to see into G-outs and switchbacks.
If most of your riding is pretty open at average speeds, Evo will be a great choice on the bars. If you're doing more XC style riding or counting grams, Portal can be a great choice on the bars, where it will also work well for gravel grinding and bikepacking. If you typically ride trails you know really well, but they're tight and twisty, Hangover on the helmet is solid. If your trails tend to be steeper, gnarlier, or faster, then Portal on the helmet will give you more punch down the trail with a bit more width at the same time.
Ultimately, having both will make the biggest difference, but if you just need one and still aren't sure after all this, email us, we want to make sure you get what's best for you from the start so you have the best experience, rather than just telling you buy the most expensive set.