And I'll say 42.5mm. The long lens will be great for portraits. 25mm is just too wide.
For landscapes, the kit is fine. You don't want or need wide aperture.
For group shots, 42.5mm will be a little too long, but shooting wide, the kit lens will at least be at f/3.5, so you'll be "only" two stops behind. Zoomed in, you'll be 3.5 stops behind with the kit. Plus, you get no motion blur hand holding 14mm for about 1/30s. At 42mm, you need to shoot about 1/90s for no motion blur. So you're more likely to need the extra speed shooting long.
That makes lot of sense to me. A 42mm at f1.7 will give me much more than a 25mm at F1.7 (3.5 stops vs. 2 stops like you said).
Say what???
Alphoid was saying that either of the f/1.7 lenses you are considering will be 2 stops faster than the kit lens at its (the kit lens) largest aperture of f/3.5, at the short end of its focal length range. When you zoom the kit lens out to its longest focal length (42mm), its fastest aperture goes down to f/5.6. At that focal length the kit lens is now 3.5 stops slower than an f/1.7 lens.
He was NOT saying that the 42.5mm at f/1.7 gives you any advantage over the 25mm f/stop-wise, as your comment suggests. At f/1.7, either lens will have the same advantage(s) over the kit lens, with respect to f/1.7 being two stops faster than f/3.5, and three and a half stops faster than f/5.6.
You need to pause and figure out what you really want the prime lens for. If the purpose is mainly to allow you to take indoor photographs of your friends and family, rooms and decorations etc. that would otherwise turn out too dark with the slow kit lens, the 25mm f/1.7 would be preferable because it has a wider angle of view and you can fit more things into the image (e.g., the family group seated on the couch, everyone seated at the dinner table, the three cousins in front of the fireplace etc). The 25mm is also a more usable focal length for outdoor walkaround use as well.
On the other hand, if your purpose is to get a lens primarily for portrait use (head and shoulder shots of one person), then the 42.5 would be preferable.
Perhaps you might want to consider buying an external flash and bounce card instead of a new lens, until you get things sorted out a bit more? That way you could get well lighted, nice looking shots indoors with the kit lens you already have. And, you'd have the flash for use with whatever lenses you end up with down the road.
Just a thought
