Go with the 15* when in doubt. You can always vent to cool down but you can't make it warmer in the wee hours of the morning. You can leave the air mat at home most trips and save a lot of weight. When the temps are warmer you can vent or when really warm just flip it upside down and compress the loft. The only penalty in warmer weather is a little more weight. You will also find that you may not need as much top insulation when it is in the 30-40's with the warmer UQ.
Since you were asking questions about a 15* or 30* UQ I had a pretty good idea of what you were looking at
but the same holds true with say a 20* or 40* UQ. If you are only going to have one, go with the 15*-20* UQ from any vendor on HF and you will be pleased.