This is why I ended up with so many airbrushes!
The gravity fed can do a slightly finer line (certainly finer than I'm capable of!) than the 150, but as they are virtually the same brush, it really is negligable. For the example you pose the 150 would be an excellent choice.
There IS a cap for the top of the 100LG, but as one oaf to another, you learn fast! The cap is a bit of a pain to use, and I leave it off except for storage.
Whatever you do decide as your final choice, do yourself a favor and order a couple of extra needles, a couple of spare head washers and some needle lube (Badger Regdab or Iwata-Medea Super Lube) for spares.
The nice thing about the 150, is if you do decide in the future that a gravity fed is a must have, the parts are all interchangable with the 100LG.