Suggestions needed for GPS Module

Hey guys,
I need your suggestions on buying a new gps module.
I'm looking for gps with high accuracy upto 2.5 meters or less and gets satellite lock fast i.e. with cold start < 35-40 secs.
Reason for asking here rather than doing comparisons myself is I already have got one fastrax up501 and it gives random positions upto 15 meters when locked on 5-6 satellites open air/outdoor, whereas data sheets say accuracy of 3-5 meters.
So I'd rather rely on experiences you guys've had rather than data sheets alone.
P.S I'm working on a project which would get distance b/w two lon/lats and both have to be within 50 meters, hence gps accuracy plays a great deal.
Any suggestions on how can I improve my current GPS accuracy would be appreciated aswell.
Cheers

..they are all the same..
With only one frequency it cant do better.
You should know that when near building, you can get reflexes (on and off).
Those nanoseconds delay is enough to give you wrong position.
There is also the fact that signals from satellites right above arrive faster than those from satellites at low angle.
(due to less distanse throu air)
GPSs that capture both freqs. can handle this by measuring difference of time of arrival for the different freqs.
(different freqs. are slowed unequally in air )