I really like splitting my batches up as it gives me an opportunity to do a true head to head comparison of yeast strains, dry hopping techniques, etc. The smaller fermenters are also easier to move and make it easier to control temperatures. I can easily fit 4 carboys in my fermentation fridge for 2 ten gal batches at a time.
That being said, 1/2 bbl sanke kegs make great fermenters for 10-12 gal batches. They allow pressurized fermentation, don't allow any light in, and you can make pressurized transfers after fermentation is complete. A cheaper alternative would be HDPE drums. My local craigslist usually has listings for drums of all sizes pretty cheap. The 15 gal ones usually go for about $10 each. Just make sure you know what it was previously used for (preferably food).