Based on cables alone, the 50' balanced interconnect is superior to the 50' speaker cable. In the case of long cable runs attenuation and noise injection are the big concerns.
Of the two problems, noise is the most problematic for interconnects. The fundamental design of the XLR circuit is based on transmitting two identical but inverted polarity signals on a pair of conductors. At the receiving end, any signal that is not on the opposite polarity conductor is considered noise and is rejected which leaves only the signal originally transmitted through the cable. Active circuits at both ends of the signal path (cable) helps alleviate the attenuation issue.
Noise is not a significant issue with speaker cable because of signal strength. It's really hard to inject enough noise on a speaker cable to create noise on the speaker end. However, attenuation can have a very noticeable affect on the quality of sound from the speaker. As the signal is affected by the attenuation (capacitance, inductance, resistance) of the conductor, the sound from the speaker is changed. Your mileage may vary on the degree of the change but speaker cables beyond 8 to 12 feet will increasingly influence the sound coming from your speakers.
Obviously it is more complicated than that but, without knowing any thing else about your system and environment, the cable question is pretty straight forward.
