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Body armour is a worthy investment particularly if you're planning on getting air (i.e. start jumping) with your landboard. If you are in a buggy less so, you are surrounded by a metal frame and consequently less exposed.
On the whole body armour is more expensive but then it's doing a lot of other things that just protecting your knees and elbows. Lumber support, spinal and coccyx are now standard features.
If you don't want to go for the full mesh suit definitely invest in some crash shorts when you're boarding to protect your bum from some duty heavy landings.