Your license will allow you to download and install any previous XenForo version that has ever been released.
However, we would have to strongly discourage that. At the end of next month, XenForo 1.5 will no longer be supported and will receive no further updates.
With XenForo 2.1 now in beta testing, and a year having passed since XenForo 2.0 became our stable, supported release, it is time to talk about our ongoing support for XenForo 1.5. XenForo 1.5 is now considered a legacy product and is no longer in active development. We will continue to support...
xenforo.com
XenForo 2.x adds (and will continue to add) additional and much improved functionality over XenForo 1.5, so we'd highly recommend embracing the current supported version.
One day those themes will fall behind modern design standards and approaches, perhaps even break in future browsers (as would be the case with any theme designed by anyone over time, and even in the default style).
When that happens, you won't have any support in getting those issues resolved because everyone will be supporting XF2.
I'd say that the improvements and benefits that XF2 brings far outweigh any reasons for remaining on XF1.
You may also want to check on which terms the @ThemeHouse themes are upgradeable. I'm not sure if you have to buy them again, whether the updates are free, or whether there's a discounted rate available. If the updates are subsidised in some way, that's definitely a no-brainer.
Actually you are not buying a "XenForo 1 License" or a "XenForo 2 License" - what you are buying is a "XenForo License" whichs gives you access and permission to use any version that has been released so far or will be released within the next 12 months.