I have two LBD 1TB SSDs that i'll use for storage in Raid, alongside a fusion 1TB that will be for the OS and Apps.
The Lacie is uberfast (already tested them with my MB Air), and obviously has way more SSD space than the fusion option.
Things to consider:
1- If you don't have particular needs in terms of having data you frequently use on SSD, the Fusion is a great option: the system will just feel super fast.
2- The LBD is very fast, but one drawback you might want to keep in mind is that it is noisy. You can hide it somewhere but you definitely are going to hear it if you keep it next to the imac (the cooling vent is always on, whether it's used or not, quite a stupid design flaw indeed).
Net, if you are not a pro photographer / video editor / musicmaker i'd go for the fusion.