I think the unupgraded version is so much better than the upgraded. +
just to add a +1/-1 is too big of a cost for such a little of a change. That said, it's a great "pinger" and buffer. Got to love versatile cards.
This is the only nymph I have (just got it 2 days ago from oracle

) and yeah, I looked at the upgraded version and decided I wouldn't upgrade it. I like being able to activate it for 3

instead of 4

, even if it goes to +6/-6 instead of +5/-5. I've been using it as a creature removal card in my rainbow deck, and it works pretty nicely, especially alongside Gravity Shield. (+5/-5 kills anything that Gravity shield can't stop)
Also notice that unupgraded, it is 1/6, and can activate on itself to make 6/1, and when it's upgraded, it improves to 1/7, and can do the same thing to itself to make it 7/1, so in both cases it has just enough hp to pump itself.