Well it's not easy to compare RailClone with Parametric Array, since our product has not been released yet. But i think that both products are very different.
PA seems to be very flexible to create complex structures, including nice features as the Maxscript support. It's well suited for advanced users, but perhaps a bit difficult to use. Also, since it's a script not a plugin, the performance may be a problem as well.
In the other hand, we are focusing RaiClone in the ease of use. We want that our users can create a complex structure as a metal & glass railing with only two clicks: select the model + click the spline, saving on this way lot of time (and money) in the project workflow. Because this, the customizable Library Browser it's one of the more important features of the plugin. With the first version, the software will include more than 75 predefined models, including materials (for several render engines) and textures. You also can create you own models and libraries in very easy way.
Although RC will offer many advanced features, the first version is focused mainly in construction over splines (railings, fences, guardrails, piping and more). In the future we will include support for surfaces and other multi-dimensional structures. Why don't we include surface support initially ? Well, because a quality question: the plugin must solve all possible cases of uni-dimensional structures before moving to the next step. However, actually the plugin is so flexible that you can even use it for cladding using nested RC objects.
I can also forward you, that RailClone won't be as cheap as PA. From my point of view, it's not possible for a company to provide the required quality support and continuous product updates, without a realistic pricing policy. Our intention is to develop a plugin that will evolve along many years, including new features with each release.