As the title says, which is nicer to own and maintain long term. Reliability? etc etc. 
I’ve seen a 16v i’m interested in but is the extra valves and paralever worth it? 
Do the cams have to be removed for clearance setting?
thanks

You do need to remove the cams to adjust the valve clearances but they very rarely need doing. In 7 years I have had to do it once in 65K,
My feelings is that if you have a good 8V then stick to it.

To me the most important difference is the front brakes which any 4V wins in spades. I think riding, not maintenance, should be the primary concern when choosing what to ride.

The 4V valves are certainly more of a PITA to change but the valves on all Ks are pretty static so not something I'd be concerned about.

And there's nothing really wrong with a 2V K100 so if you're happy with it then there's no immediate need to upgrade.

The wider wheels on the 16V bike allow you to mount radials which aren't available for the narrower rims.  The radials, to me, track better on rain grooved pavement, and there is more grooving done every year around here.  Also, because of the wheel size, the seat height of the 16V is a bit lower than earlier models so if you are short of leg, you will be more comfortable with your feet down.

On the other hand, the exhaust is subject to cracking which can be expensive to repair.  +1 on the front brakes.  They're awesome.

If you have 3 spoke wheels on a 2V you can use radial tires. (I do on all of my 2Vs.)

110/80-18 front
150/70-17 rear (or 140/80-17 if you can find it in a radial)

thanks for the insights.

Actually, they're almost completely different bikes, except the body (fairing, tank, saddle, etc.)
The frame, the fork, the brakes, the wheels, the suspension, the transmission, the fuel injection system, the exhaust, the foot pegs are different from a k100 8v and 16v. And, of course, the engine (head, pistons, crankshaft, minor details).
The 8v is simplier, very reliable, lighter and slightly less powerful (-10cv). Cheap budget to run, tyres are small and unexpensive.
The 16v is more complex, also very reliable but with a maintenance more expensive, weights more, but offers a better roadhandling due to the strenghened frame, much better brakes, and more comfort, mainly due to the paralever. Its longer wheelbase is better for hi-speed,  long curves and for comfort.

Yet, I prefer the 8v engine, for its smooth and constant behavior. The 16v is prone to mid-range erratic issues, and you need to be over 5000 rmp to enjoy its features.
I think that all the qualities of the 16v are more enjoyable with K1100 versions (LT or RS), and the 8v fits more the K100, specially in the basic version, very pleasant with its low weight.