00p,
Both are good candidates for aging in an oak barrel and there are no hard and fast rules as to how long you should age...it's all about personal preference.
A few things to think about...
1) You mentioned you're borrowing the barrel from a friend...each time a barrel is used for aging the characteristics of the barrel change. And the more you use the barrel the longer it will take to fuse the unique oak characteristics of the barrel into your beer, until eventually the barrel is really of no more use.
2) To add a little more insight...if I'm not drinking beer, I'm drinking bourbon and I've brewed 5 batches which I have aged in cask bourbon barrels. 3 batches w/ Makers and 2 batches with Woodford. I also have a New Holland cask waiting just waiting for me to add a batch.
So, that being said, when using oak casks I enjoy a nuanced RIS flavor profile....hints of oak, vanilla, chocolate and the warming influences of bourbon so I tend to actively age for 2 to 4 weeks before pulling it off the oak. If you go much longer than that, the oak will start to take over, covering up some of the finer elements of a perfectly brewed RIS.
With the Makers, I go straight to keg after the aging and start drinking as soon as it's carbonated. With the Woodford, I again go straight to keg, but let it sit for about 8 weeks before I start drinking.
If you're doing a Barlywine, the maltiness of the brew would definitely benefit from sitting on the oak just a bit longer. Not necessarily 6 months, as 6 months is a very long time for beer to sit on oak and you're definitely going to get a very oak forward flavor profile.
One of the primary reasons high gravity beers tend to be aged longer is because of the stress the yeast are under during primary fermentation with so much alcohol present toward the end of the fermentation, which tends to produce an undesirable amount of off flavors. Now with the highly specialized and high quality strains of yeast being produced the need to age extensively has been significantly reduced...so don't sweat the aging process, just sample every 4 to 5 days and carb whenever you think it's to your liking. After carbing try a few now, then try a few more every couple of weeks to form your own opinion.
And if you want to experiment cheaply, just use oak chips/cubes instead of oak barrels...no one will be able to tell the difference. And you'll be able to perfect your recipe!
Happy brewing and hope this helps...