OK, I'm relatively new myself, but I'll give it a go. I'm sure others that are more experienced will chime in shortly as well.
I've played all that you have listed outside of the the Kala and I think that they are all very good values. The first thing that you may want to try to decide on is what scale you like best. I think that you can do almost anything you want with either size you decide upon. I thought that tenors are what I needed to play because I'm a pretty big guy, but, the more I play, the more I find myself gravitating toward concert size. I find that I can strum and pick on the concert just as easily as I can on the tenor and it's just physically more comfortable to play. The only way you're going to know for sure what's right for you is to play both, make a decision and stick with it. Since you're in SF, Aloha Warehouse in Japantown might be a good place to visit as they have both the Islander and Ponos you're considering. If you are able to make it down to San Jose, you can check out Ukulele Source on 5th street as they have the Koalana and at least the Mainland concert. So, I know it stinks to have someone tell you to play them and make a decision when you are asking for advice, but that's the only way to know for sure.
Having said all of that, here are a few of my brief thoughts on what I have experience with:
Koalana concert - Nice bright, traditional ukulele tone with quite a bit of volume. The neck is a little thin for me from front to back, but playability was still easy enough. Overall aesthetics were not as pleasing to me as some of the other choices, but that is very subjective. There are quite a few positive reviews of the Koalana here on the forum so you should be able to find additional info pretty easily.
Mainlands - I owned a mahogany tenor for a very brief period. It really was a great ukulele, but I found a good deal on a LoPrinzi that I couldn't pass up and that made the Mainland expendable. I really liked the look and sound of the Cedar tenor more than the Mahogany, but there weren't any available at the retailer where I bought mine and the Mahogany was still nice. I went back and forth between the tenors and concerts before I bought the one I had. The tenor had a little more presence and bass than the concerts, but I was stuck on the idea that I had to have the tenor because I was a big guy. Looking back, if I would've bought the concert, I probably would still own a Mainland. The sound was still bright, but not quite on the same level as the Koalana. The neck definitely felt more chunky and fit my hand better, but nut width felt a little narrower.
Islander (tenor only) - Feels and sounds relatively similar to my Kanilea. Nice wide neck with good thickness and plenty of room to move around. It actually sounded a little more "muffled" than some of the other ukes I've played and was not nearly as bright and ringing. Lots of volume on tap and a very solid feel to it.
Ponos - My favorite of your choices. I actually just picked up a MC at Aloha Warehouse yesterday (even though I wasn't going to buy any more ukes for the time being!
) The necks on Ponos are my favorites at the moment. They are very chunky and just fit my hand perfectly. Soundwise, they seem to be a little quieter than others I've played, but the tone is excellent (IMO). They don't sound as bright and plinky as some, but they don't sound muffled to me either. Just nice and balanced and perfect for my ears at the moment. They do seem to be a little neck heavy/body light compared to other brands I've tried (maybe due to the heavy duty neck and truss rod?), but not so much that I have any problems with them. Definitely my favorite ukulele at the moment.
Whew, that was long-winded, but I hope it helps.