Sharkskin is one of the brands of plastic underlayment, they are lighter and stronger than felt paper. The synthetics don't wrinkle up when they get wet so they are great for those situations where the shingles won't be installed immediately. I just re-built my roof including new rafters, sheathing and skylights. I used titanium udl 30 which is another brand of synthetic because I worked alone and knew it would take me quite a while to finish the shingling. I really like the titanium, it lays nice and flat and has lots of lines and markings to use for alignment, it also has a great non skid texture.
I added ridge vent just in case the house needs more ventilation with the new roof.
For a normal quick roofing job I'd probably have just used 30 lb felt, it doesn't really matter what's on there once the shingles are in place.
The titanium costs about $14 per square, I don't know what 30 lb felt is selling for now days, but if it is half the price I'd save the money, if it is closer to 75 percent I might go with titanium just because it is lighter to work with and easier to get straight( at least for an amatuer like me) . My previous roof had 30 lb felt and lasted 25 years so don't worry about it if that's what you choose.
15 lb felt is very fragile so you will have to take more care if you use it...DaveB