Need help picking an entry level bike

[brigada]

Hi all! My future trail buddy is getting a bike this month and I'm trying to help her out as if it were my own. So here are some basics:

This will be her first non-Walmart kind of bike. She's 5'4 (1.63m) and the budget is set at around $600-650. We settled on it because it is good enough for her to feel the difference and not too huge of a loss if she doesn't fall in love with trail riding after all.

I suggested this Cannondale Trail 6 yesterday and she ended up liking it lots:
http://www.cannondale.com/2013/bikes/women-s/hardtail/trail/trail-women-s-6

It's priced at $630 and it would be great if she could get 5 or 10% off for paying in cash. The parts aren't perhaps the best bang for the buck, but it's still better than the similarly priced entry Myka, for example. I figure, if it fits her well, she can always upgrade the parts later on and if not, she shouldn't have trouble selling a Cannondale.

My only concern is the frame size. None of the shops have it in stock in small (15in),only medium (16.3in). Now I'm thinking that M might actually fit even better given her height, but this is where I really need advice from more experienced riders. Thank you all in advance! :)

[brigada]

I should have noted that we've been looking at bikes for her for about two months now. The situation with used bikes is quite disappointing because the appropriately sized bikes don't seem to appear in local ads too often and they tend to be overpriced and in need of some serious maintenance. She's very adamant (and rightfully so) about not buying anything suspicious that may have been stolen.

As for the store bikes, we had to rule out Treks and Giants due to unreasonable pricing. Jamis has good deals, but they only stock bikes in larger sizes unfortunately. Same for several other brands we've looked at.

There's a Focus Donna 4.0 I suggested for $450, but the crankset seems like it will fall apart on the first ride and the v-brakes on it do not instill trust :/

Seems like mission impossible to get reasonable pricing and good sizing in the same deal! :D

Specialized has a 15.5in 2012 Rockhopper on sale for $760, that sounds interesting as well but I think she'd really have to fall in love with it at first sight to make such a budget jump.
Same for Cube Access WLS. A lovely bike, but even further out of her budget at $800.

[thekarens]

For what it's worth both my partner and I ride a 15. She's 5'5 and I'm 5'3, but we have the same inseam and my arms are a lot longer. Our inseam is 30 inches.

[brigada]

Thank you for the info!

Nothing has been bought yet, she decided to wait until she has some more money so that she doesn't blow all her savings on a bike. Meanwhile I've been looking around (can't help it :D) and noticed this 16in Haibike: http://cikloberza.com/show.php?id=82111
It comes with full Deore + XT rear derailleur and Rock Shox Tora SL. At this price (380 EUR, or $500) I think it's a really good deal. It hasn't sold for a couple of months so I think the seller might even agree to knock the price down a bit. I hope it isn't stolen so I definitely think getting the seller to sign a sales contract would be wise if she goes for it.