I too had your problem recently, i had all my other ultralight kit and was fairly happy with it, the only item i couldn't decide on was a sleeping mat. I was going to go for the peak elite ac for exactly the same reasons as you, but decided to give the multimat adverturer a go at 170g it was the lightest close cell mat i could find. Compared to a inflatable mat it is bulky, but i just lash it to the bottom of my MLD Prophet rucksack with not problems. I have just come back from doing the West Highland Way in Scotland, where i rain non stop for 6 days, but i had no problems with the mat although when i did get it at first, i admit i did think it looked far to thin to be affective, but i was wrong, it was fine.
What i intend to do is purchase the Peak Elite AC pad and use it in slightly colder weather periods but stick to the multimat when the temperatures are slightly higher. Like most pieces of kit, it really is about trial and error and personal preferences. Good luck with your choices and as someone who has done his first ultralight hike i promise you it is well worth it, i was amazed at the difference when i finished each night, i just felt so fresh compared to walking with heavier packs and my feet sure appreciated the difference, no dreaded blisters for a change.