I personally dont see the value in getting the plus model with the inbuilt Mac Valve, I have one on my airhose so it doesnt matter what brush i use i have that control. I'd rather spend the extra money on supplies
As
@MarcosD has stated, the C cup would rarely be filled for what you would use a micron for and if you are doing a large area then you just top up the smaller cup as needed.
I have the CM-B & CM-SB and at my skill level i cant tell the difference between the .23 and .18 and for me it wasnt about 'pulling finer lines' it was more about the better atomisation at low pressure. I can honestly say that they were the best investment i've made and I hear a lot of people say that the weight of the side cup is an issue, but i can say i didnt notice a difference, its not as if its the equivilant weight of another airbrush on the side.
As for your concern about needing to half fill the side cup that is not the case as has already been said, you can literally put just a drop or two in there as it feed into the bottom of the cup and is drawn up the side of the cup to the brush. You can also get different size side cups to use so if you are frequently topping up the cup then maybe a larger cup may be useful for those odd occasions.
One thing you may not have thought of is that the Side feed allows you to change colours with relative ease as long as your paint in the cup is below the level of the tube that sits into the airbrush.
just get a side feed bottle and fill it with your chosen cleaner, remove your current paint cup, insert the cleaning bottle, spray out into a pot and then replace with a second cup with your next colour.
the cup that comes with the brush doesnt have a lid, but the next size up does so i've bought several of those for the times that i'm wanting to use multiple colours without having to 'waste' what is in there.
Ultimately the choice will be yours and like you I was trying to figure out what would be best and eveyone seemed to really like the side feed and when i first got it i couldnt see why everyone was so keen on them but once you get use to it they are great. my only 'issue' was with the cleaning as i was so use to cleaning the gravity feeds and this seemed so foriegn.
A lot of members here will quote an Eclipse CS and a Micron of some description and its true, with those two brushes you would have 95% of your needs covered.
Sorry this turned into a really long post, but i got a little carried away