In terms of the center vs inline pump head the only difference is which would be a better fit for your setup. Both temps take in the water to the center of the pump for circulation. Two options are offered for convenience.
The advantages of stainless steel over plastic are numerous so I'll keep it as simple as possible.
-- Plastic, while potentially food grade, will wear over time and potentially leak substances into your fluid. For me personally (and our team here at love2brew) we would never want to have our fluids running through a plastic piece.
-- Stainless Steel is significantly more durable than plastic and will hold up much longer over time. Purchasing a pump is really an investment versus a throwaway purchase; it makes sense to spend a little more now and never have to worry about it than have headaches in a few years.
On a side note the reason we chose to carry Chugger Pumps over other competitive brands is that their model is COMPLETELY food grade; including the propeller inside of the pump. Other popular models do not have a food grade propeller (constructed with glass resins); definitely not something I'd want to use on a product myself or my friends/loved ones would be consuming.
If you have any questions please let us know!