Is it better to realize the loss and buy back in at the lower price or just hold it?

I'm not a big fan of hypothetical questions. It depends on what you're trying to optimize for, i.e. taxes or total return. Most investment advisors would tell you "don't let the tax tail wag the investment dog."

There's an opportunity cost to selling with the intent to buy back in at a lower price. What if it recovers as quickly as it dropped? What if it drops further?

Time in the market generally beats timing the market. If you liked the stock at $150 and intend to hold it indefinitely, you should love it at $100 and consider buying more instead of selling.

Adding my comment to the answer:

You can't realistically assume that you're selling and buying at the same price.

Even if you did, what's the point? If you're selling/buying near instantaneously the loss will be disallowed due to the wash sale rule, which prohibits selling an investment for a loss and replacing it with the same or a "substantially identical" investment 30 days before or after the sale.

If you wait long enough, it's virtually impossible to buy back in at the same price. At a minimum, you're crossing the bid-ask spread.

You have multiple issues buried within this question. First, we don't know your tax bracket. For my answer, I'll assume 25%. This simply means that in 2016, you'll have a taxable $37,650 or higher.

The interesting thing is that losses and gains are treated differently. A 25%er's long term gain is taxed at 15%, yet losses, up to $3000, can offset ordinary income. This sets the stage for strategic tax loss harvesting. In the linked article, I offered a look at how the strategy would have resulted in the awful 2000-2009 decade producing a slight gain (1%, not great, of course) vs the near 10% loss the S&P suffered over that time. This was by taking losses in down years, and capturing long term gains when positive (and not using a carried loss).

Back to you - a 15%er's long term gain tax is zero. So using a gain to offset a loss makes little sense. Just as creating a loss to offset the gain. The bottom line? Enjoy the loss, up to $3000 against your income, and only take gains when there's no loss. This advice is all superseded by my rule "Don't let the tax tail wag the investing dog." For individual stocks, I would never suggest a transaction for tax purposes. You keep good stocks, you sell bad ones. Sell a stock to take a short term loss only to have it recover in the 30 day waiting period just once, and you'll learn that lesson. Learn it here for free, don't make that mistake at your own expense.