What tonearm will better mine?

Hi "noromance",

the Jelco is a very good tonearm as it is.

The above mentioned Ortofon is a Jelco OEM-product and will
not better the one you have significantly.

But you should consider two already mentioned tweaks plus an
extra one:

1. The Ammonite collar is a very fine tuning and will help stabilizing
    the Jelco. If you buy one you should order the M6 screw (serial
    is M5) WITH the nylon tip. That prevents harming the Jelco shaft.

2. For the Decca I would think about a slightly damping of your
    tonearm and using a good ebony headshell from Yamamoto
    or Uwe Bretschneider.
    http://anamightysound.com/shop/headshells-en/uwe-n-o-s-headshells/

3. I have modified my Jelco 12" with a phono cable which goes directly
    from the SME-connector to the phono stage.

With these tweaks my Jelco is in the same league with some of the
best recommended tonarms (Reed, Graham, SME, DaVinci...).

Op, my advise is simple. If you like your sound, don't worry about what the others say and be happy.  Our hobby is endless. :)  Always pushing for a better sound and to what end.  I use a Jelco 750D tonearm on a restored Thorens 160 Super.  It's a great arm, I see no point in spending 2-3x the money to get little improvement.  Trust your ears! Good luck! 

bdp24, Wow ! I´m glad you ordered that superb T3Pro linear TA. 
Mine is also made with an uninterrupted run of silver wire to RCA plugs.
I´m also planning to try silver Eichmann plugs. Are yours the same (or equivalent ) as these on eBay:
http://www.ebay.co.uk/itm/KLE-Innovations-Silver-Harmony-RCA-Plugs-Set-of-4-/131438981244?hash=item1...

Rewire the Jelco and buy a drop weight and be done.