Keep in mind that the stack measurement listed for the new SL7 is WITHOUT the top bearing cover (which you have to use), so add 10mm to the listed number to get the actual stack.
Then seat tube and head tube angles should not be used to determine the sizing. With ST, you should always have your saddle set to the same position, in relation to the BB, so it should not be used to judge reach (i.e. you can't make up for excessive reach, by simply pushing your saddle forward). It should only be used to judge how much setback will be needed, and if you can achieve the desired setback.
HT angle will affect reach slightly, but only if you need to run a ton of spacers
Based on the stack numbers, I'd say go for the 58. But, you also have to consider the wheelbase, front center, and trail numbers as those will have a big impact on the handling/stability of the bike...and vary quite a bite between the two sizes.
So, a 61 with a -12 may actually be the best route for you.