We received some of the new Form Labs tough resin last week and have just run a couple of tests using Ember. First impressions are that it is stronger than PR48, we will try an quantify this later. We've printed the Spark logo in PR48 and the tough resin, the tough resin still has a similar flex to PR48 but it seems less brittle.
Tips on printing - as with their casting resin, over-fill the resin tray a bit, 150ml of resin should be OK. The default exposure for PR48 at 0.025 resolution is 2.5 sec, this works but we seem to get better results with a slightly longer exposure, the most recent prints we used 2.7 sec.