the 17" also has a 1.25GHz processor compared to 1 GHz for the 15".
But if you are sure you don't need the Superdrive, i guess you will be okay with the 15", because these are the only two differences, the speed and the DVD writing. Note: you can still watch DVD on the 15", just cannot write a DVD.
the 15" is 1024x768 while the 17" is 1440x900 resolution. as ionas mentions, check what will work for you. i suggest you go the store and play with the machine just to check the resolution on screen, everything else is the same. i for one, would not have minded the lesser resolution, but i needed the superdrive. remember, these lcd screens don't do reduced resolutions (less than their optimal) nicely.
you could buy the 15", spend the spare cash to max out the RAM, and be happy for the next few years, then you could buy the G6 (?) 😉