Which cartridge should I upgrade to now?

I second Dynavector.  A much better match for Rega arms.  If you can stretch a little to the 20x2, you'd have your choice between low output and high output MC versions.

For consideration: I've seen many RB300 and RB600 arms come and go here on A'gon for well under $1000...would be a drop in replacement/upgrade for your RB250.

FWIW, I also agree that the Denon 103 would be out-of-sorts on a Rega arm. Because of it's stiff compliance, it really thrives with a much heavier arm.

I'll second the recommendation for the Audiomods tonearm. I upgraded from a RB250 to the Audiomods and have been extremely pleased with the improvements. One of the great audio bargains IMO.  

Got the Audiomods Classic 11 arm. Great. Improvements appeared subtle at first, more apparent with a bit of time. Not long after had a little accident with my Shelter 501 mk 11 cartridge. Found a just retipped one on evil bay. It's just bliss now, for me anyway. 

Thanks for all your suggestions

Glad to hear it all worked out for you. When you have time and budget you should send your old Shelter 501 to be retipped and you will have a spare.

Houseofhits - lots of good advice, but here's some additional info related to a couple of the above posts

Option 1: A very good "Denon" match for the Rega/Audiomods arm is the Denon 103 or 103R from ZU. - they offer a a couple of different "grades" of 103(r) cartridge based on channel tolerances and the aluminum shell gives it the added mass required.

Option 2: Having said that - send your existing cartridge to Soundsmith for cantilever replacement + stylus retip. They offer several stylus profile and cantilever options.

A word of caution; the more "advanced the stylus profile - the more important it is to have the cartridge setup perfect.

Do not use the printable protractors because they are not accurate enough - I used the Mint Best protractor - but there are others available that are comparable.
http://mintlp.com/best/9-series.htm

I have the stock 103 and also purchased a re-tipped 103 from Soundsmith - the retip’d version was significantly better across the board.

I epoxied a brass shim to both cartridges to increase the mass - it worked wonders...
http://image99.net/blog/files/23c020f75290d3392577113371f4dc94-38.html

Does your arm have the KLE Innovations RCA’s? If not try them. I have the Absolute Harmony RCA’s - They are superb at conveying the delicate signals. But they do require some burn-in.
http://image99.net/blog/files/a8494e8645dea3d750d109f819103099-61.html

BTW - I also have the Series II Audiomods arm - fantastic!!!  :-)

Regards - Steve.