Which one gives you the better impression and why?
Galvantula, by quite a lot. It's appearance reminds me of a cooler Ariados, which already looked great, and I like spiders anyway.
Which Dream World art stands out the most to you?
Galvantula has a very odd appearance on it's own, so it's DW art stands out the most to me. Honestly, though, I'm not the best guy to ask about DW, because I don't really use it.
If you could be one of these Pokemon, which one would you want to turn into?
Scaling vertical walls, keeping people away with electrified webs, having 6 eyes, and looking awesome are only some of the benifits that becoming a Galvantula would herald. Definitely Galvantula.
Against one another, which one would win the battle?
Sure, Zebstrika has Flame Charge. However, it has poor stats that make it frail to just about any attack that hits it. Galvantula has Compoundeyes, giving it great accuracy to go along with it's moveset. A fight between these two could go either way, really, but I'd have to, unfortunately, say Zebstrika wins here. If Flame Charge misses, though, then Galvantula would likely win.
Zebstrika or Galvantula?
Definitely Galvantula. I like insects/arachnids in general, and I really, really, really dislike Zebstrika's design. It also helps that Galvantula has one of the best designs in Generation 5.