Power and heart rate during long efforts

Hey crew, i'm getting stuck back into some long efforts a couple of times a week eg;

2 x 30 minutes at 300w
3 x 20 minutes at 300w

I usually do a session like this during the week and again on the weekend if no racing. Is it better to do 2 longer ones or 3 of the shorter ones?

Also regarding heart rate, what is an acceptable amount of drift? When doing the 20's my ave hear rate def increases from say 135bpm for the first one then into the mid 140s, and the last is usually averaging about 150bpm sometimes higher.