If you can only afford one filter right now go for the 500.
I agree, FWIW. A 250 Hz filter is really useful in contests, with cheek-to-jowl signals. For most work, 500 Hz is fine.
An IF filter is _way_ better than AF DSP. It is placed _inside_ the AGC loop, so strong signals outside the passband don't affect the receiver's gain.
One thing you could try to work a weak signal that's near a strong signal (I think the TS-480 has a narrow AF DSP filter):
. . . Set a narrow bandwidth on the AF DSP filter (250 Hz or so);
. . . Center the weak signal in the AF filter's passband;
. . . Turn off the AGC;
. . . Reduce the RF Gain so that the strong signal doesn't overload the receiver.
You may be able to get good copy on the weak signal. I've run many RTTY contests, and normally have AGC _off_ to stop the strong signals from modulating the weak ones.