The pro-oiler was my first (and only) oiler (on my honda hornet 600), but when i bought a new bike, i didnt build it over due to difficult settings, some strange malfunctions (after 4 years use) & (for me) hard to build in and switched to spraycans.
One of the nicest features is it takes any oil, even regular motoroil, so no expensive refills here
Scottoiler has the Esystem, an electroniccal controlled system, which seems to be the evolved version of the pro oiler and it looks nice, the display with G-force-indicator is nice for gadget-loving-nerds like me

but the oilcontainer seems rather small, at least on a longer motorbikeholiday you'll have to refill, or buy the additional lubetube.
A cheaper solution is the original scottoiler (MK1), or the new model the Vsystem. Both work by attaching something to the bike-engine so when the bike is running a vacuum is resolved so the oiler starts dripping oil , the amount of dripping can be managed offcourse.
These versions can also be fitted with the lubetube.
Original scottoiler oil is advertised, since with "gravity" systems different oils result in more/less drips.
In the netherlands the OSCO oiler (
Osco chainoiler - Home) is very popular (but the english translation doesn't work on that site?)(
english review) , and is a combination between the on-bike-build of other oilers, and non-automatic-spraying of the spraycans.
Every other 200 miles or so you pull on a knob and the chain gets oiled, you decide when it gets oiled. (best is just before tankstop of before getting home/@hotel, since rthe chain is nicely hot and oil has some time to settle in)
Very simple in use & building on your bike, no electrical wires or attaching to valves and therefore almost foolproof
Anyway, I would choose the scottoiler Esystem cause i am a gadget nerd