The answer is both. The Steem Power (SP) of the main account and the amount of SP delegated both increase with the interest.
When you delegate Steem Power to another user what you are actually delegating is the underlying VESTS (commonly referred to as MVests since the numbers are huge).
Interest is applied to all holdings of SP by a slow increase in the SP / MVests ratio.
So when you delegate SP the interest will accrue to your own holding of Steem Power but will also increase the amount of Steem Power delegated to other users. The underlying MVests (both your amount and the amount delegated) are unchanged.
You can see this by looking at the wallet page of a large account like freedom. The SP and delegated SP holdings increase a little each time you refresh.
However when the delegation is returned, the original amount of MVests is returned which will be equal to the amount of SP delegated plus the interest earned over the delegation period. So the delegated account will benefit from increasing SP over time while they have the delegation but will not retain the amount of interest accrued.
The amount of interest are small so generally it's not a consideration for most users.
[Note that this discussion is of the "interest" inherent in the Steem system, not any interest payments from users to whom you delegate the SP].
The word "Delegation" simply means entrusting someone something while still in total control of that something.
When you delegate Steem Power, the delegatee is treated as a proxy for the delegator by the blockchain. He gets most of the benefits that comes along with it(increase upvote worth and bandwidth). However, the delegator remains the OWNER and holds TOTAL control of that delegated SP and thus it makes sense that the interests are only for him, the owner, not the proxy.
It's worth to note though that the Delegated Steem Power still increases over time. If you delegated 1000SP, that will slowly increase depending on the interest rate of the OWNER of the SP. After a month, you might see that it's now at 1002SP. The increase 2SP still and will always be ownes by the delegator though.
Yes you do get interest from your steem power. Even if you had delegated those sp to someone else, you are the one earning the interests, and not the delegatee. To be more precise, it is your vests that is earning interest. With every block created on the blockchain, you earn a tiny bit of vests from your stakes.
I know this from my own experience. Months ago, I delegated 1250sp to some account. Today, I can go to my wallet or using any available tool, I can see that the delegated sp is now 1255.22sp. That’s an increase of 5.22sp over a few months.
Read more in the technical details from this awesome post by lukestokes https://steemit.com/steem/@lukestokes/if-you-hold-steem-power-you-re-earning-steem-automatically
You don't get interest from it. It's like giving money out that you know you can get back, whatsoever the person does with the money ain't your business but you can always get it back anytime. This means no interest to the person that delegates out. It is only the person that gets the delegation that can benefit more.
If you delegate Steem Power, it is no longer yours for as long as it stays with the person whom you've given. And so, you do not get interest on a delegated Steem Power. The person who gets the delegated Steem Power gets whatever interests or rewards from it.