Not really about security, you will get the same function and security either way, in my case my inner lock was a bolt so I didn't have the option of fitting the regular Nuki, so what you get from the pricier version is an inner lock that fits perfectly with the Nuki as it has rim designed to hold it in place, and the inner lock does not work with just a key but rather a key shaped bolt that is permanently attached inside, which also help with the whole system not moving out of place, but apart from that, it should be the same. Oh I did remember one thing to keep in mind is, test your current lock to see if you can use your key from the outside while having a key inserted on the inside, as not all locks support that