I have and use both. Guided, for me, takes longer than freehand overall. It is slower to start, but once the initial grinding is done, subsequent progressions tend to go faster than freehand.
It tends to be more precise in that I can set a specific angle and it stays that way. The guided system I use is better for awkward cutters that make it tough to securely grip and move on a stone, and I won't use any powered sharpener without a guide unless I'm sharpening a garden tool, axe, or lawnmower blade. Most often I only use it when resetting bevels for commercial work, and then only on high dollar knives - even then I sometimes will opt to do the entire thing by hand. I finish everything off by hand.
That said, for myself I sharpen freehand 99.5% of the time. I do not see any real difference in quality of the edges produced excepting the guided ones can make a more attractive edge cosmetically. They also tend to look worse if the edge has a lot of warps or high/low spots in the primary grind, so the enforced precision can be a boon or a curse.
For personal sharpening, freehand is ultimately the way to go unless you have some disability or severe lack of manual dexterity. It is faster, easier to translate across a variety of cutting tools, easier to improvise when needed, relaxing, rewarding, huge variety of potential stones/surfaces etc etc. It is not a simple skill to learn at the higher levels of competency, but relatively easy to learn at the medium levels - more than enough for most uses.
That said, it is a skill, there is a learning curve, there will be set-backs along with improvements. If you want to learn a skill, go freehand. If you just want to avoid the hassle of learning and are OK with being restricted to a piece of equipment to produce a sharp knife, get a guided system.
