I think there are two - actually, three - different things to consider to answer this. The Server, and Client, and the Network between the two. On all three, I think the answer is that statics are definitely more efficient, but to varying degrees. Of course, this efficiency is offset by a tradeoff in the loss of creative flexibility. Such is the nature of this type of thing with computers
First, the very, very easy one: Network related, there isn't even a question. Statics don't have to be sent over the network, dynamic items do. In fact, as far as I'm concerned, this discussion can end with this point alone; The difference with this one point alone is big enough to be the only factor that you balance against creative flexibility. Quite simply; having lots of dynamic items around causes lots of extra bandwidth usage. On slow connections, or in places with a huge number of items (such as trying to build a house with dynamic items, tile-by-tile), people can see a noticeable difference in network lag, and it will pump up your overall bandwidth usage.
Second, to POL itself; As I noted above, I don't think this is that important, but I do think that there's a clear advantage here to statics. First, less data is stored on statics, meaning less memory is used. Second, the very fact that statics are, well, -static- means they don't change while the server is running, so POL can store the information on them more efficiently. It's not a coincidence that POL's statics on 096+ are stored in a configuration file; a topic about which we've talked before. On the other hand, dynamic items are just that - dynamic. Meaning POL must allow for those objects to be altered, added and deleted. This fact alone means dynamic items will be slower and use more memory. However - not much. I think the difference, while clear, isn't important at all, especially when it's effect is contrasted to the effect on network bandwidth, and when you compare it's negatives to the benefits of creative flexibility.
Third is the client; Again, I think that the answer is clear, but also even less important than the answer for the server; As before, these items are 'static', and the client knows it never needs to change them in any way, once they are loaded. That, alone, makes them more efficient than dynamic items. But again, I think the difference is very minor.
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Basically, I think as long as you have a reasonably good network connection (not using a standard consumer DSL or Cable modem, for example, but a true broadband connection with roughly a Meg of available bandwidth in both directions per 50 players, or so), you should not worry about it at all, and keep all your decorations dynamic, so that they can be changed... but
build everything static.
Not sure if any of my players read this site, but we're getting close to being done anyway, so I'll just be short, and hope if they read this, they'll keep their big mouths shut.. hehe.. but we've been doing massive statics work - basically starting with a 'clean slate'; There's more to it than that, but not that's relevant for this discussion. What we've been doing, though, is basically building all the buildings and structures as statics, but leaving decorative items out completely of the statics. That is, there are no furniture, tables, counters, plants... nothing at all but walls, floors, roofs, windows, etc; This way, player owners of houses and the leaders of towns will be able to completely decorate as they see fit.
Some balance will need to be maintained so no one goes overboard, but our systems already put caps on how much decoration can be done by home owners and town leaders, so that's not a big deal. But since we have plenty of memory and bandwidth for the server, it is going to work out just fine.