This one is easy... the Exynos is easily the better chip.
CPU
Snapdragon 600: quad core krait 300.
krait 300 has an IPC (instructions per clock) about 0.7 - 0.8 that of a ARM cortex A15. Remember Qualcomm gave up performance for efficiency.
Exynos Octa: quad core A15 + quad core A7 (big.little architecture).
you get the power sipping benefits of the A7s with the top end performance of the A15. The price you pay for this is in die size, i.e. costs more.
Conclusion: if the two chips have the same clock speed, which current rumors seem to show, then the Exynos will be 20-30% faster from a compute perspective.
GPU
Snapdragon 600: Adreno 320.
same GPU that is found in the Qualcomm S4 Pro with an increased clock (estimated 15-20% increase in performance)
Exynos Octa: Imagination SGX 544MP3
Pretty much same chip as found in iPhone 5 (with added DX10 support) but clocked at 533MHz instead of 300MHz like the iPhone, nearly double the clock.
Conclusion: The iPhone 5 GPU already beat the Adreno 320 of the S4 Pro by a good margin in most benchmarks. With the large increase to clock speed the Octa should easily take this category as well.
References
Exynos Octa:
AnandTech - Samsung Details Exynos 5 Octa Architecture & Power at ISSCC '13
Snapdragon 600:
AnandTech - Qualcomm's Next-Gen Krait 400 & Krait 300 Announced in Snapdragon 800 & 600 SoCs
GPU comparison, offscreen tests are most telling since they normalize to common resolution (compare nexus 4 (adreno 320) to Iphone 5 (sgx 543MP3)):
AnandTech - Google Nexus 4 Review - Google's new Flagship