Realistically - desktop VR is significantly easier than standalone devices to develop a shippable product right now if you're new to VR. Standalone headsets like the Focus Plus have tons of benefits from being inherently standalone but they're mobile products and the current generation of standalones are using Snapdragon 835's. Porting or developing for standalone requires an specific skill-set. Here is a video that highlights some helpful tips. You can definitely start for Focus but you'll be able to get going faster with desktop.
Desktop VR on the other hand doesn't have the same headroom limitations - you have a dedicated GPU to handle geometry and shading. You can get away with alot and most asset store assets are geared towards desktop. The desktop development ecosystem has been around longer and there's a more robust set of tools, forum posts, and learning resources, ect.... The install base of desktop HMDs are also higher than standalone currently (but there are more developers targeting that market). If you're developing VR you'll need a gaming PC with a powerful GPU which is the primary barrier that prevents people from getting a desktop VR - you're more than halfway there already.
Each platform has limitations and use-case. It's possible to develop your project in a way that will work with both standalone and desktop using intelligent just know that optimization for standalone is no joke and is resource intensive whereas desktop doesn't require the same focus on optimization and you'll be able to iterate much quicker and have many more tools and computer resources at your disposal.
Here are my current top recommended resources for beginners: