well, flash is more reliable and generally gives a greater battery life. however, they are generally a smaller capacity. the highest you can get 32 gb (i only of know apple and creative who do that) however, these are very expensive, $300 or more.
I presume you are a audiophile, (why else would you be on this forum). in that case i suggest a hard drive mp3 player to hold all the lossless files. unless of course you have less than 30 gigs of music and ware not looking to expand your collection, or do not use loss less. in terms of battery, there really is no need to worry, because its not exactly going to run out during the middle of the day, and so you can always charge it every night.
Also, although hard drives are meant to be less reliable, i have never had any problems with them, and considering most people use apples ipods, which have hard drives, you can see, there is no problem. (but i seriously suggest you don't get an ipod... just don't. i am sure you already know that, but just to reiterate, do not get an ipod)
O, i forgot to mention, if size is important, then you need flash. hard drives will jest not do.