I thought I'd miss a qwerty keyboard but trust me you won't! Both phones are fantastic, and a lot of your decision depends on UI, or User Interface. Samsung uses Touchwiz and HTC uses Sense; a lot of people hate Sense but I honestly think I like it more than Touchwiz because it gives you a lot more options for personalizing things. I have the Samsung Galaxy S II now and I'm probably going to sell it on ebay when the One S comes out and get it instead - the snapdragon S4 is blowing away the competition and leaves nothing to be desired.
Bottom line is: go to a store and try them both out, I bet you'll like how the One S feels better in your hand since the GSII is a little bigger and feels more plasticky... Everyone has their opinions, but then again, you asked for ours so that's how I feel! Either way you're getting a fantastic phone, and if you can get a killer deal on either one I say go for it!
Haven't had my hands on a One S but I did have an HTC Amaze 4G for a while.
I used an Amaze 4G for about a month after using a GS2 for about a month or 2 so I did have plenty of time to use each phone form an opinion.
So from that perspective, FWIW, my recommendation is the GS2.
I liked the Touchwiz interface better than the Sense interface.
Text was easier to read on the GS2. Fonts were larger and darker.
I thought the menus were simpler to navigate.
I thought it was easier to find things.
The Amaze 4G is touted as having the best camera in a smartphone but honestly I thought the pictures and video from my GS2 were just as good if not better.
Don't get me wrong. HTC makes good phones and the Amaze 4G is an awesome phone. If I hadn't used a GS2 before using the Amaze it would be in my pocket right now. I just liked the GS2 better.
Again, I haven't seen or used a One S. I'm just giving you my opinion based on my experience with the Amaze 4G.
Your mileage may vary.
The HTC One S will be the newer phone. Newer phone usually means, newer and better technology being added. (Galaxy Nexus wasn't IMO except the OS)
It will come with Android 4.0 Ice Cream Sandwich aka ICS probably well before the Galaxy S II sees it (or this could be a the date the update is released by T-Mobile and Samsung as a marketing ploy to sale more Galaxy S II's)
I'm sticking with my Galaxy S II as I'm not a huge HTC fan. Here's a little points breakdown. The newer device as expected with newer devices edges out the Galaxy S II specs wise but you can't go wrong either way. They both will have their share of problems and followers.
HTC One S
Newer faster processor +1
Newest OS +1
Unibody design (can't change battery) -1
No micro SD slot -1
1280x720p Display +1
Beats Audio +1
Galaxy S II
Has a proven track record and was awarded the best Android/Smartphone by several repuable websites +1
Micro SD slot +1 ( I have a 64GB micro SD card in mine for a total of 80Gb)
Slow updates by Tmobile/Samung -1 (but only 3% of devices have Android 4.0) +1 or 0 here.
400x800 Display -1 (Same as orginal Galaxy S even though it looks way better)
One correction on what Animaleyezz said, the One S does not have a 720p display (that's the One X), it has a qHD display. What Spoon said about checking each out in the store goes double for the display, as each is different and it's hard to say which you'd like better. The One S will definately have the more powerful processor and from reviews has great battery life. But so does the GS2. For me, the lack of an SD slot on the One S is a big negative, but if you don't plan on storing a lot of music or video on your phone, then it's not too bad. The biggest plus for the One S is the camera. I don't think it'll be beat by anything for a while. It's hard to say, really, which I'd recommend. The GS2 is a great phone and I love it. The One S has some tantalizing specs and features, too.
But to throw a real monkey wrench into the deal, Samsung may have an anouncement about the Galaxy S III by the time the One S comes out!