Id have to agree, ride both and see which one feels better. Just seeing your sizes Id lean more to the 58cm. Im 5'10" and I ride a 56 but I have 'unusual' measurements haha.

I'm 6'2" and on a 56 tri bike, but that's so I can get the absurd drop I like (~20 cm) with a relatively tall seat tube at a slack-ish 76.5 deg effective angle. Are you borrowing a road bike or tri? If it's a road bike, you'll be able to get by with either and just adjust to the needs of the riding position. Same for a tri bike, but if the bike is too long front to back, you may find it uncomfortable or impossible to get your elbows at the desirable 90 deg bend when in the aero bars. If you are an "average" 6' (i.e. ~33" inseam) either bike will give you plenty of stand over clearance and you can fit yourself to the top tube without any problems.

I'm 5'11" and I ride a 56. You can "make" most bikes fit, but the closer you start to the "best" size the easier and more comfortable. They tried me on the 58, and said the 56 was my size. Since I have no clue, I went with their expert advice!!

For what it is worth --6'3" here and both my road (and NEW) tri bike are 58cm.

There are no real rules of thumb because we all have different body dimensions. I'm 5'11", but have the inseam of someone who is closer to 6'3" & a freakishly short torso, so I ride a 56 with the seat way up. Go by feel. Since you said they both feel the same, you're probably fine on either for now, IMO.