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You might try riding the Rondanee XL frame just to make sure. It is too bad they don't make a larger fame, because in my opinion it has the best gearing for touring of the three. If a 520 60 mm frame fits you, the extra large Rondanee might also fit.

The Surly builds larger frames than 60 cm. That could be a plus if you fit better on a 62 mm. The top tubes on the LHT are on the long side for a given frame size. For me this is a disadvantage, because I have very long legs in proportion to my torso. This may not be an issue for you.

In my opinion the wheels on the Surly are a little better than on the 520: slightly better hubs, spokes, and rims. The LHT also comes with the steerer tube uncut. This allows a little more bar height adjustment, if you tell the shop not to cut it until you get the fit dialed in. The XT rear derailleur on the LHT is higher in the Shimano echelon than the Deore on the 520. Either one would get you across the country. However, the front Derailleur on the 520 is a few steps above the Sora FD on the LHT. This is one item I might consider changing on the LHT. I'd take the 3403 Sora FD off and replace it with a 4503 Tiagra.

I think the big one for me is the ability to change the chainrings on the Shimano 4 arm M543 crankset on the 520 to a 44/32/22 relatively inexpensively. Conversely, the 5 arm Andel crankset on the LHT can only go down to a 24 tooth inner chainring. My daughter's stock LHT has the Andel crankset, and I replaced the stock 26 tooth inner ring to a 24 tooth for her. I'm willing to bet that before your trip is over you would be willing to pay the $60 to have those lower gears. I changed our bikes out from the 48/36/26 crankset to 44/32/22 as soon as we got home from our cross country trip. It was money very well spent. I only wished I had spent it earlier

Gearing is about a wash on the LHT and the 520. IMO the gearing on both stock bikes are too high for loaded touring in hilly or mountainous areas. You might talk to the folks where you buy your bike and ask them to replace the xx-32 rear cassette with an xx-34. They will usually do this free, and every little bit helps.

The 520 comes with a rear rack, but the price difference between the bikes more than offsets this.

I have a LHT, but have only test ridden the 520, but the 520 seemed a little more nimble and felt lighter (only perception). My LHT was built from the frame up so my opinion on components has to be taken with a grain of salt. Nothing on my bike is on a stock LHT, except the frame. Not that my components are any better, just different. Both bikes have very good reputations, and you can't go wrong with either one. If you can test ride both bikes enough to discern a difference, fit should be the deciding factor.

Good luck.

P.S. If all else fails and the decision of which bike to choose is close, pick the one with the color you like

Good luck.