Longer speaker cables or phono cables?

In this situation you're in the "cage" or "gridlock". Phono cables are thin and have high per/unit resistance. Making them longer will certainly increase it thus decreasing the cartridge output to the phonostage very often with big deal of dynamics loss and even details. If I knew your amp I would tell you more precisely if you can use long and thick speaker cables. Usually low-powered zero feedback amps can oscilate driving the long and thick runs. For high current amps it's no problem.

I must agree that the longer speaker cables are certainly the lesser of two evils.
One way around this might be to place your phono stage (if separate) adjacent to the turntable, thus driving the interconnects at much higher levels & buffered to boot. This would help reduce the deleterious effects of long cable reactance (capacitance / inductance).
Frankly I see no problem with the longer speaker cables; I've always used them myself.

You have the current to drive longer speaker cables. Long phono cables will add more capacitance and noise.

Keep the phono leads short to the preamp then long IC's to the amp and finally shortest possible speaker wires.

Even if you use low-capacitance interconnects, the longer speaker cable is still preferred. I did an extensive study of this. The results are independent of actual failure criteria, but I needed to choose an arbitrary one. The simulation results are posted at the Audio FAQ page at:
http://www.empiricalaudio.com