Unbounce is nice too for A/B testing your landing page.
There's one thing to remind regarding a "coming soon" app's landing page: it should become later the real landing page for your app, once your app is live. So if possible, the domain / url has to be the definitive one. If you do that, then you can start SEO stuff early on with your landing page (and maybe some PR preview stuff during your Beta), and this page will already be searchable when you launch your app + you won't have to do all the SEO from scratch.
Regarding fine tuning your app's landing page and the related text (with A/B tests, for example), there's also a strong benefit: real A/B test on appstores is not really possible, so it's very hard (and long) to find what converts well. If you do that stuff on your landing page and then your app page, you can optimize and hopefully find the proper assets (icon, screenshots, text, ...) to get a much better install conversion on appstores directly at app launch.

Hope it helps!
Alex J