I am thinking about buying a Parker 100 myself.
I am down to one Sonnet now having traded one and lost one to my wife, who loves hers. My comment with the Sonnet is that the nibs are variable, in fact the best nibs I have found are the Gold Plated Steel nibs. My wife however loves the 18k gold medium on her Laque Black Sonnet.
In terms of drying out, yes they do, the cap is not sealed due to the new regulations and like any open nib pen, if you leave it on the desk for longer than 30 mins it does take a bit of starting. Having said all that, I use a Sonnet along with a cartridge 61 as travelling pens and they are both 100% reliable in that regard.
The big advantage I find with the Sonnet is that they are dead easy to clean as they come apart so easily, well unless the nib gets frozen in there with Penman Sapphire as happened a while back, but that was my fault. I can take it apart in minutes, even down to taking the nib assembly out. I discovered this when I bought a spare nib and the whole thing just unscrews. Soak it all in water overnight, rinse and dry and it goes back together ready to be filled with whatever ink you fancy. Mine does prefer a free flowing ink I find, but all said and done, I reckon the Sonnet is a highly underated pen. Not going to set the World on fire, but then again, it won't let you down either!
I was interested in Apollo's comments about the 100, JM Lewertoski has them on eBay in the Blue/Gold for a reasonable price, so I am tempted to get one, either that or somebody will go a tempt me with a double jewel "51" and the pen budget gets blown again for a couple of months
So my reckoning is go for either, better still get both as they seem to complement each other!
Jim