Don't listen to the people who diss on the leatherette...do yourself a favor and check it out yourself. It's a great option if you want black. I have the black leatherette in my 335is and I love it. It still looks brand spanking new after 1 year with ~18K miles. I like how the leatherette looks so tightly stretched over the seat frame without any of the 'sag' that leather seats start to develop after some time. I had a 335 with leather prior to the 335is and that started to show a bit of wear/sag even at around 15K miles. Likewise for the leather seats in our EX35. The leather on my M3 with 130K miles is a sight to see...it looks awful, even though I took good care of it the entire time.
And as for leather breathing better...I think that's just BS. If it's hot out there...you're going to end up with a sweaty back no matter what seats you have...well, unless it's cloth. The leatherette in my 335is makes me sweat no worse than the leather in my M3 or our EX35.