This question (and similar ones) seem to come up regularly on this forum.
They are quite different products.
Vray is a non-biased renderer.
This means it has realistic lighting, reflections (ray traced), material and shadow properties. This realism comes at the expense of rendering speed, but is essential if your aim is to assess colour, light and materials in a realistic way.
Lumion is a biased renderer.
This means it utilises technical shortcuts for simulating lighting and material appearances.
The image outputs may look similar, but to an architect , industrial or interior designer they could be very important differences eg in the amount of light reaching a space.
Unbiased renderers like Lumion are actually so fast that they can allow you to “walk around” inside the rendered environment, completed with all shadows, lighting, and even animations being rendered in “real time”. This gives an excellent user experience for your customers and can be very immersive (especially with VR).
You can’t do that with Vray because it renders frame by frame. You may be talking 1-2 minutes per frame (or more), as opposed to 0.03 to 2 seconds per frame for Lumion. You can, however, render 1000s frames in Thea and make a walkthrough video, though it will take some time (overnight maybe).
So, other than the unbiased/biased thing, why would you choose either?
My opinion is that Thea, Vray (and similar products) are ideal if you create your 3d objects and details within sketchup, meaning your materials are good quality, colours are accurate and your objects are exactly as you specify…maybe from a manufacturer catalogue. This workflow means you will create and edit everything within Sketchup and render frames or image sequences as required, make design tweaks back on sketchup, then re-render, etc. The renderer mostly just applies lighting and a few extra properties for materials. If you already have your library of materials and components well sorted in sketchup then maybe you dont need Lumion’s expensive asset library?
Lumion (and similar products such as Twinmotion and Enscape) can import your assets but PC hardware has limits to how much detail you can add before they start to struggle with the grahics load. To maintain their realtime rendering speed Lumion utilises an asset library of plants, furniture, people etc that optimised for speed…and can look a bit cartoony you might say. However this workflow is excellent if you want to keep your SketchUp scene basic (like good enough to produce technical drawings) and then quickly add the detailed 3d plants, furniture, materials, people, etc inside the Lumion renderer environment.
Of course this workflow also means that if you were to spend 3 days modelling a scene then 1 day setting it up perfectly in Lumion, then you went and changed your sketchup model so that your landscape maybe got modified with something like a new large retaining wall, then you may spend almost another day re-setting up all the objects in Lumion.
Further, if you have set up all of your materials and 3d objects in Lumion, how then can you create documents from that scene using Layout?..outputs like planting plans, scaled perspective views and cross sections? Putting a scene into Lumion is pretty much a one way workflow.
Its worthwhile remembering too that Sketchup can’t animate things (the extensions that enabled this have been discontinued) whereas Lumion can - eg trees, people, traffic, etc. That’s potentially important if you’re doing video outputs with moving trees, birds, people, etc.
A notable competitor to Lumion is Twimotion which is cheaper, has fewer settings and tools, and doesnt have as good a library of components - but is easier to import 3d components from online sources (including animated versions). Enscape may also be worth a look as it is similar and targetted more to the architectural profession.
In terms of raw speed, I believe Indigo and Octane are the fastest to produce realistic and natural looking images.
Non biased renderer can run on a more modest PC or laptop and can also make use of a remote rendering workstation PC which could either be somewhere in your office, or anywhere in the world. Realtime renderers need your PC to have a top tier video card rtx2070 or better). I wouldn’t run lumion on a laptop because it would likely overheat.