I am a 5'8" tall female. Which hybrid or fitness style frame size would most likely fit me better, 17" or 19?"
This depends highly on the particular model of the bike (which is due to factors like (effective) top tube length, bottom bracket height, wheel size, standover height, etc.) and your individual geometry (inseam, upper body length, arm length, leg length) and riding style (upright, prone, etc.) - the numbers don't really mean anything out of context. Even in the same class of bikes, you may go for different sizes (for example, sizing for a Specialized Sirrus may be different than for a Trek 7.4 fx (essentially flat bar road bikes). And if you listen to the marketing people, things like the specialized tricross/awol and trek crossrip are also fitness bikes (but are essentially drop bar commuter cyclocross bikes - totally different geometry). There are also bikes with hardtail mountain bike influenced geometries, such as the Trek 8.3 ds and Specialized Crosstrail. Even if you go with similar bikes (like the Trek 7._ fx series and Specialized Sirrus series), there are geometry changes which are important (like effective top tube length) for a given size. On top of that, a lot of frames come in women's geometries (marked WSD or similar or given a different model number, like the Specialized Vita being a variation of their Sirrus line for women), which don't necessarily size with their normal counterparts. The results also certainly don't carry over for different styles of riding (and thus, different styles of hybrid/fitness bikes) (my commuter, an old rigid MTB isn't the same size as my road bike, an 80s Panasonic). For example, I have a bike which is a size larger than most people of my height due to my relatively long arms and legs. Due to the number of variables, its hard (impossible?) to size a bike over the internet unless you have had a bike fit and know enough about bike geometries and your riding style and your geometry to make an educated guess.
You should go to your local bike shop and ask for a bike fitting to determine the appropriate style frame and setup for the bike, and make sure to take it out on a decent test ride. They will take some measurements, plug it into a formula and get some way to set the bike (the formulas are guidelines which work for most people - they aren't exactly science). Then, they'll tweak it until it works for you. You may need to swap out things like stems or saddles or stuff on a correct sized frame (there may be more than one size which can be made to fit) in order to get it working properly. On top of that, there are a lot of "women's geometry" parts and frames which are purported to fit women riders better. You may want to mix and match these parts to a regular frame or vice versa.
If you cannot afford a bike fit (they cost some money, and take a few hours), there are some videos on youtube you can look up (but really, they will not substitute for testing the bike or a good bike fit). Usually, they'll at least get you set up with a guesstimate on something that works though, for free.
The following links are good reading: http://sheldonbrown.com/frame-sizing.html http://www.peterwhitecycles.com/fitting.htm http://www.coloradocyclist.com/BikeFit/index.cfm http://sheldonbrown.com/kops.html (Watch out for this if you're watching youtube)
Its not that complicated. Stand over the top tube of the bike you are interested in with both feet on flat ground. If you have about an inch of clearance, that bike should fit you fine -- then its just a matter of adjusting seat and handlebar height and maybe riding a few different bikes that "fit" like this to see which one feels better to you.
Probably 17" but it depends on more than your height. More important is your inseam and your reach. You could be 5'8" and have very long legs in which case you probably want a 19", or you could be 5'8" with short legs and a long torso in which case you probably want a 17". The best bet, as always, is to go to a bike shop that will do a proper fit testing and let you try some different sizes. The right size is the bike that's most comfortable for you and there's really no way for anyone on the internet to tell you what that is based on your height.