I didn't buy the penultimate so I'm not sure how much my opinion really counts...but I have messed around with it in the store. I love the precision and low maintenance qualities of the pen, but I prefer my liner to be more bold and black so I like fluidline. Also, there is color variety with fluidline. The one downfall about fluidline, IMO, is that is does dry up too soon. Mine usually dries up about 1/3 of the way through the pot. This only happens for blacktrack, though. All of my other colors seem to hold up much better. I think you have more flexibility with fluidline because depending on the look you are going for, you can pick a different brush. I personally prefer the 266 because it's so versatile due to the angled tip, but I know lots of ppl love the 208. So, you have options which is nice.
One of my favorite eyeliner finds has been the Loreal Lineur Intense with the brush tip in Black. It's super duper black, even darker than blacktrack, and it dries to a shiny finish if that's something you are interested in. Downfall is it will sting if you get it in your eye, therefore is DEF not acceptable for the waterline or anywhere near your inner corner, etc.