The SP is known to be a better reg than the Sherwood for sure (nothing against the Sherwood since I used one for over a yr long time ago), but SP is more expensive too, as long as you don't buy anything from a generic manufacturer, you should be ok, all brand regs these days are very reliable and would breath well at any recreational depth, the diferences can be noticed after 100ft, less than 100ft allmost any brand name reg will do ok.
If you got the money for the more expensive stuff, you won't be dissapointed, regs, with a few exceptions, is often in the rule of you got what you pay for.
Also make sure you don't buy something that you don't need, for example, I don't need a reg that won't freeze, since I don't dive in cold water, if the feature is there, then it's ok, but my choice isn't based on that feature in particular, same with all other features.
Whenever I'm on the market for a expensive piece of equipment, I do my homework and research everything I can before going to the LDS, then I start asking specific questions and most of the time the salesman doesn't have a clue of what I'm asking him so he leaves the choice up to me.
Sometimes, I get lucky and the salesman is a equipment specialist or technician and I learn some techinical aspects of the gear I want and maybe consider another angle that I didn't saw.
My advise would be to do your homework and research as much as you can before confronting the salesman, then decide if the guy knows what he's talking about or if he's trying to spoof you with gear on stock.
I've seen many times a local LDS owner make the kill with one of the newbies that land in his store and sold them $1,200.00 worth of gear for over $2,000.00, later, when the diver retires, he buys back the gear barely used for less than $1,000.00, needless to say, I only do fills there as a last resort.
I'll go $34.95. 
