If you want a silver slide, the best option is hard chrome.
Depending on who does it, you can have a choice of final look ranging from the original satin gray, to a stainless steel look, to a bright mirror polish. It all depends on how the metal is processed before the plating is done.
Hard chrome is a very hard finish, and is very rust resistant. It's proof against any solvent you might find outside of some dangerous industrial application, and is proof against many acids.
It's actually harder than a Swiss file so it's one of the few gun finishes that's really a lifetime finish.
Because of it's "slick" finish, fouling doesn't stick to it like to plain steel, it's much easier to clean, and it requires less lubrication.
When a customer asks for a really durable finish, most custom gunsmiths recommend hard chrome. It's still the best all-around gun finish.
Most of the platers offer special pricing for parts like slides.
Here's the top platers. Read their info closely. prices will vary depending on what you want.
Note that some use brand names, but it's all hard chrome.
http://www.apwcogan.com/
http://www.fordsguns.com/
http://www.armoloyftworth.com/Armoloy_WebDevelopment/Web_Pages/Firearms_Applications.htm
http://www.originalmetaloy.com/
http://www.techplate.com/firearms_pl.htm
http://www.mahovskysmetalife.com/