No, she isn't into tech, and she won't be envious of my phone vs. her phone. I don't think she will care. What I will care about is if she can't do some of the things I want her to do if her phone doesn't support it. But, at the same time, I don't have $150 just laying around to spend recklessly.
Id say get the incredible either way. If she finds its an overkill of a device then sell it on ebay for $500+. Compare that to the current average ebay selling price of an eris is ~$220.
If you decide to sell the incredible then you will have $500. Subtract the $200 you paid initially(this is all assuming she will be buying phone subsidized). You know have $300. Buy an eris off of ebay for $220.
You get what you wanted and profit ~$80.
Then again you are going to have to wait anyways Incredible shipments have stalled. If you wait till mid July there should be a whole new batch of Android phones hitting the market for verizon. Two supposed hgih end motorola devices.
I'd say best bet is too wait out till mid July to see what's actually on the market. If at that time you still decide on an eris, the eris's price will probably have plummeted to the $100-$140 range.