I'm being asked by many people including Lindens about comparing Mesh objects with Prim objects. The more that i've learnt about making Mesh, the more I've come to realise that comparing is very difficult, i'm starting to wonder if you can actually compare Mesh with Prim objects.
From the start the idea was to create objects that cost less prims and yet are more detailed than a PRIM version, after all Mesh is supposed to be more efficient and better than Prims right? well yes and no…
The Prim is a simple creature. It has a set shape, set amount of sides, set numerical count. You rez one prim, and the land counts it as 1. You link it to another prim and they count as a 2 prim object. The world is built on the safe knowledge that everything is made of prims. Even sculpts are just smoothed off prims.
Mesh on the other hand is a wyrd creature compared to the familiar prim. It does not have a set shape, does not have a set numerical count. You rez a mesh and the viewer tries to calculate a Prim Equivalence (PE) so it can conform to the current system and this PE is effected by a number of factors - Size, Physical Weight, Number of Textures or wether it has a script inside.
If you Link two Mesh together the PE may increase or decrease depending on factors of the linked mesh. If you link a normal PRIM to a mesh, it may not even seem to count towards the Mesh's PE.
A Mesh Objects PE is not set in stone either. You could have an object made with 5 Prims and have a Mesh version that comes in at 4 PE, if you enlarge the Mesh version by a metre or so, it's PE could jump to 8 PE. The Bigger your mesh the more PE it costs. It's a bit confusing and really kills off mesh for large scale use not to mention griefing potential of an object that takes up prim count when enlarging.
Physics weight also is impossible to compare to Prims as it is not a set thing. For example i have an Airship which is 24 prims. I left it at 24 because i wanted as many people as i could to sit on it and Physical Vehicles only allow 30 Prims to move. Each Prim has a physical weight of 1, each avatar has a physical weight of 1, this is how it's always been from the start, simples.
With Mesh though you have the ability to set the Mesh Physics to your desired shape and size and the Physics weight is calculated from that. I built my Mesh version of my Airship and i added some extra detail and textured to the best of my ability. There is NO denying that the New Mesh airship is far more detailed and better textured than the Prim/sculpty version. Yet it calculated to being 38 PE compared to the 24 prims. 'Oh No, its far to many prims to be a physical Vehicle! ' , actually NO, because although the 24 Prim Airship has a physical weight of 24, the 38 PE mesh Airship has a Physical weight of just 5!. Unfortunately Land counts the PE so we will see Physical Vehicles with a PE way above the standard 30 Prims, so if you don't have a lot more than 30 prims left on your land, you can expect problems with Mesh Vehicles Crossing.
People will understandably compare Mesh objects to Prims based on Prim count alone and i think this is a bad habit to get into.