I have both, and wouldn't part with either, but if I HAD to choose, I'd keep STLE. That said, the code is way cooler on STTNG. It's FULL of custom callouts etc, and they've done an amazing job of differentiating the various modes. You know what you're getting there, as you already have it.
Code on STLE is in progress. It's getting better, but nobody knows what we'll finally end up with. They've announced custom callouts recorded by Karl Urban... but no date announced for release. The first few updates came quite quickly, and were big improvements, but it's still got a ways to go.
Right now the modes are a little too similar in the first level. Too many shots to make, and not much differentiation. That said, 1st level modes are fun.
I think the lighting and audio on STLE are top notch. It's a VERY impressive game. But it's not the classic that STTNG is... yet. It's really close though. A little code tweaking, and the addition of custom callouts, and this will be an amazing title. As is, it's great.
I honestly couldn't choose between them, but I think if I HAD to, I'd keep STLE. Maybe that's just because I've had STTNG for almost 15 years though.