The linear rails impart greater rigidity — the tradeoff is one of maintenance:
- the linear rails of a pro would require on-going maintenance in terms of lubrication, see: Rail and linear guide maintenance
- the Delrin V wheels of an SO4 are maintenance free for day-to-day usage (or non-usage), simply requiring that things be kept clean, but will eventually require replacement when worn
In the event of a really bad crash, the Delrin V wheels sometimes give way, and if so, are easily and inexpensively replaced — for a Pro this might involve damage to the block/bearings or possibly the endmill or the belts (but no one has crashed/damaged one to that degree that I’ve been made aware of that I can recall).