The Spraque 715's are a Polypropylene type, and polypropylene are generally accepted as the best in amplifier electronics circles.  
(mojo debate aside)  
 
Polypropylene are the type I prefer (it's an opinion thing) to me they are more "tonally pleasing" but then it's because I prefer sonic clairity and accuracy. I use them in high gain Marshall 2203/4 type amps and prefer the tubes and OT to color and create my tone. I would recommend these if you are looking for sonic clarity. Some have said they sound "sterile" but these are usually folks who are used to fuzzy old caps, I say they sound "clear" they let through more of your guitars real tone. Most high-end instrument amps use them.  
 
Polypropylene types:  
Sprague 715P, and 716P series (aka "Orange Drop")  
Illinois Capacitor  
Xicon PF series  
 
Polyester film/foil types are generally regarded in guitar amplifier circles as being the "most like" the original mustard caps found in vintage musical instrument amplifiers. If you're looking for mojo or caps that have sonic coloration then polyester foil are the ones for you. Some amps need these caps to achieve their desired tone, like if you were building/fixing a Tweed or Marshall Bluesbreaker you would surely want to use these polyester foil types.  
 
Polyester types:  
Mallory 150 series  
Cornell/Dubilier DME series  
 
Note: old vintage amps used polyester foil as they were the best price-vs-quality capacitors available back then, when polypropylene caps became available they were considered too expensive to use back in the day. Also many manufacturers (like Marshall) were in fact using war-surplus parts which were readily available back in the 1950/60's.  
 
WIMA MPK4 series are polypropylene and physically known as "Box" type caps which are intended for use on PCB's (Printed Circuit Boards) as opposed to axial lead types like Mallory 150 or Illinois. Be aware the lead spacing may not be correct for your PCB (measure the lead spacing of caps you are replacing) and there is often not enough extra lead to do a bend-to-fit job.  
 
The Xicon and Sprague are officially "radial" lead types but are physically large enough that the leads can usually span across circuit boards using point-to-point wired "turret" (Marshall) or "ring" (Fender) style lugs.  
 
Mouser carries the Sprague, Mallory 150's, and Xicon (though you have to do a search on thier site for the Mallory's) and Antique Electronic Supply is great place to get 400 and 600 volt caps, and they have the Illinois at: http://www.tubesandmore.com/  
 
If you are undecided, try them both and decide for yourself, at $0.75-$1.30 (from Mouser) it's not really too expensive to try them both and at least you would know. If you try them both, record yourself playing your favorite tune/lick using each type so you can compare after-the-fact, it's much harder to compare tone while you are playing because the non-audible touch/feel/resonse can vary and that often will psycologically change one's opinion about real (audible) tone.  
(what you hear when you play -vs- what others hear when you play)
 
3/22/2004 3:33 PM
Bradster2k 	Sorry, I omitted the Sprague's polyester type from the list:  
 
Polyester types:  
Mallory 150 series  
Cornell/Dubilier DME series  
Sprague 225P series (aka "Orange Drop")  
 
Yes, Sprague Orange-Drop's come in both Polypropylene and Polyester types so it helps to know which type a person is speaking of when they're describing tone.