For reasons of humidity control and security, I would go for a wooden viv with sliding glass doors every time - admittedly it does not offer the same range of viewing angles for looking at your snake, but a wooden viv is also easiest to secure light fittings to, and it's very easy to secure shelves and climbing branches. The doors themselves can be secured against escape and prying hands with cabinet locks.
I have a two-storey, glass-fronted viv housing my corn snake and my ball python. The UTHs are inside the vivs, but you can only do this if you use a thermostat and thermometer to accurately control the temperature of the UTH. They are covered with substrate and I have not had any problems. It's easy enough to drill a small hole for the cord and thermostat sensor, and remove the plug from the UTH to thread it through.
As regards material, most commercially available vivs (at least over here in the UK) seem to be made out of melamine-coated chipboard; I know a guy who builds his own and he uses either Medium Density Fibreboard (MDF) or thick plywood (the viv I now use is one of his). You can also get moulded plastic units, which are generally used for species with higher humidity requirements where a wooden viv would rot.
In your case, I would go with a melamine viv long enough to accomodate your snake for the rest of its life, as you don't want to be upgrading again in the future.
Hope that helps...
Simon
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0.1.0 normal corn - "Slinky" | 0.0.1 normal ball python - "Humbug"