I am 5'10" and my bike is a 58cm. I 'should' have a 56cm but they didn't have one. What this means in actuality is that the stem (100mm) is probably 20cm too long for optimum comfort. I'll change it someday. Maybe. It's really not that big a deal. I DOUBT that at 6' that a 58cm is 'too big'. Anyway, it isn't really about the seat tube length. The seat tube of a LG Crosstrail is 50cm. The seat tube of a 58cm Diverge is 52cm. That is correct. What is sold as a 58cm only has a seat tube 52cm long because it is the 'effective seat tube length' that is relevant. The top tube on both bikes slope, but the top tube of a Crosstrail slopes much more than the top tube of a Diverge.

The 'stand-over height' of the 'large' size bike of each model will be very different. What is more important to you as a rider is the 'reach' to the bars. The 'reach' of the LG Crosstrail and the 58cm Diverge will (should be) close. I will assume the salesman eyeballed a working seat height for you and sent you out to test ride. I don't mean to be mean, but when I go in to test ride a bike I tell the salesman what height to put the seat at. The cheapest Diverge is double the price of my roadie. You don't want to leave it to random knuckle draggers like me who have nothing better to do than post online. I have to agree with the earlier poster who suggested reading threads on bike fit. But even better is to find some YouTube videos on bike fit where the information is presented clearly without the usual arguments and disagreements on a user forum. Good luck.