What tonearm will better mine?

If the Garrard truly is as "rough in upper-mids & treble" and "lacking in detail" as many experienced users suggest

I don't think you need to worry about that. 

@noromance

Chakster, nice set-up. Thanks for tge info. What improvements would there be over the Jelco with my Decca SG-P?

I’ve never tried Japanese Jelco, but German build quality of Schick is superb, i like oldschool style of design and simplicity. The Garrard 301 Hammertone is even more attractive to my eyes when it comes with Schick tonearm. I could only dream about Garrard Hammertone, but i have Schick on my SP-10mk2 with UltraCraft shell and low compliance Argent MC500HS cartridge since i’ve sold Ortofon SPU Spirit and Classic GM mk2. Anyway, it’s hard to ignore some other tonearms i wish to try, but Jelco is not in my list. The good thing about Schick is the low price and high quality, you don’t have to pay much to have your own experience with Schick "12 on your Garrard. I have several tonearms at the moment, for the Schick (with two different counterweights) i would ask $1800 including shipping worldwide and pp fee (just to give you the raw idea how the Schick compete with other suggested arms in terms of price).  

Meaning no disrespect, and acknowledging that other posters may in all likelihood have more experience with various arms than do I, may I humbly suggest that the extreme nature of the Decca/London cartridges is what needs to be taken into consideration when looking at arms for use with them. They put out a massive amount of physical energy, revealing any resonances in the arm's tube and rattles in it's bearings. The freedom from rattling bearings is one reason unipivots have traditionally been recommended for them, the other being that unipivots often feature damping, which the cartridge can use. The cartridge has vertical and horizontal compliances differing from one another, making getting the resonant frequency in the optimum 10Hz range in both planes more difficult than do "normal" cartridges. 

I heard a Decca in a VPI 3D....outstanding.

Thanks all. No one has actually explained what sort of specific improvements one should expect from a new (expensive) arm. I think I'll order the low-slung counterweight and carbon fiber headshell for the Jelco for the moment and spend the difference on more records.