(try as I might, I can't get the hang of Worktroll's boiling-water-and-exacto-knife method of cutting and reposing plastics without them looking battle damaged and/or being commissions from Jack the Ripper ::) )
Don't use exactos, or those break-off blade knives - they're too light, the blade flexes, and it's harder to control cutting. Try a Stanley knife
And consider a razor saw
I confess I ignore the handle, I prefer the direct control. With the stanley, make sure you have no flesh in the line of cutting (pro tip! We bleed inside over our minis, keep it there ;) ), and use a firm rocking action. With the razor saw, just pull it back & forth.
If the blade has any flex in it, that's where I find my problems starting. Stanley knives should be in any hardware store, most hobby stores will have razor saws.
Plastic vs. metal? Plastics are just so much easier. Metal multi-part? Some are intelligently designed to be easy - the resculpt Uller is a great one. But the main culprits IMHO are poorly thought out tiny contact points, and multi-part legs. I'll use the reseen Archer as a great example of this done well (mainly) - feet, legs and lower torso one piece, in a dynamic pose, upper torso & arms separate, so you can pose however you want without major hassle. The only downside are the pestilential missile covers, and that one lump on the back which is separate. Bad? The reseen Thud - multi-part legs which are irrelevant because the feet on the base prevents any actual posing, plus rigid arms.
Executive summary: plastic, I can paint right away, or mod to my hearts content if I want to. Metal, I may be forced to spend much time assembling, and modding can be surprisingly hard even with multi-parts - eg. I just modded a Shadow Cat to running mode over the last week, only to find the upper torso no longer sits on the lower :(
Reality? Plastic for 'core' minis, IWM continues to facilitate a wider range not cost-effective for plastic. We get choice.
W.
