Either would work fine. Kegs are nice for storage since there is no light exposure, and no oxidation taking place if you properly purge the headspace with CO2 and set the seals.
Carboys can store beer very well also (especially the glass ones), as long as you minimize exposure by filling towards the tapered neck. You do need to make sure the airlocks do not dry out, but you only need to check on them once per month. Put them in a dark place, or wrap a towel around them to minimize exposure to light.
I would say both can store beer for up to a year with no problems if you follow the tips above. I have an RIS that I plan to age in a carboy for 5-6 months minimum.
Primary #1: Surly Bender clone #2:Berry Rhubarb Wine
Secondary #1: Empty #2: Empty #3: Empty #4: Sparkling Elderflower Wine
Kegged: Dark Side Dunkel, Irish Red
Bottles: Big 50 Barleywine, Framboise Lambic, Dark Belgian Strong, Belgian Golden Strong, Kicked by a Moose Scotch Ale, Apfelwein, Russian Imperial Stout, Barolo Wine
On Deck: Spruced Warmer, CDA