To be honest, I don't keep up any more with all the new receivers so, I don't know all the features of all of them. I could not find any info about the Explorist 515. Only 510 is shown on Magellan's website.
510's upside is a decent size screen and a built in camera which may be nice for recording geotagged finds.
Downside is; it does not seem to have a built in magnetic compass. I'm not sure about adding custom maps to it.
For years now, I've been using mostly Garmin receivers and developed personal preference for them.
Between the Oregon and Dakota, I'd go for Oregon because when using maps, screen size is VERY important.
There is a slew of ready, good maps that can be used on Garmin plus, virtually any map (including any historical scanned paper maps) can be added using Custom Map feature. http://www.garmin.com/us/products/onthetrail/custommaps
It can also double for car navigation so, even that I don't have all the info on Magellan,
my personal choice (from the three) would be Oregon 450T
YMMV
HTH