I require a 6" 1500# ball valve the current datasheet specification gives the seat material as REINFORCED TEFLON(PTFE + 15% FIBER GLASS).

This valve is now obsolete and I'm being offered 2 differing types of replacement.  One of these has Devlon seats (from reading up I believe this is a suitable replacement for the RPTFE seats).  The other replacement is using Nylon seats - this is substantially lower in cost however, is there an issue using Nylon seats for 1500# application?

Reinforced PTFE (RPTFE) typically is not used in Class 1500 valves because when it is subjected to constant tensile stress or pressure loads, PTFE will escape, i.e. flow, at room temperatures. This behavior is called "cold flow" and is contingent upon the pressure or tensile stress exerted, the duration of this stress as well as temperature.

There are several grades of Devlon thermoplastics.  The most popular being Devlon V, which is a proprietary blend of Nylon 6 and Nylon 12 manufactured by the James Walker/Devol Engineering company in Scotland.

Specifying "nylon" on a data sheet is very common, and it permits the valve manufacturer the flexibility of selecting the specific grade of nylon for both the service and the seat design.

Nylon is generally inert to most chemicals, but is hydroscopic (that is, will absorb water) so it may not be a good selection for wet gas or water/seawater service.

As mentioned by Camvalve, RTFE is not commonly used at Class 1500 anymore due to problems with those pressures and typically alternatives are Nylon or Devlon V. 

My general preference would be for Devlon V as it is a specific and relatively high grade of Nylon material which likely has better performance characteristics than many of the other Nylon grades in use. 

If this is sour service, you may want to consult the supplier/manufacturer for confirmation that the material is suitable for your specific service environment. The literature does indicate some significant limitations on Nylon in sour service but there is very little published data publicly available on Devlon V and sour service (at least that I have found).

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