Personally, I would choose a slightly different topic where the differences are more substantial. Staying in the computer realm, you might consider arguing for/against a Windows PC versus a Mac with OS X.
But if you stick to the Dell topic, here are some points:
1) Their phone support for consumer products is terrible. You get shunted to someone in India who gives you a fake american-style name and is trying to simulate a mock american-style accent (I have nothing against India, I love the country, it's the phony act that I hate. Dell should give it up; let the Indians sound Indian). The people you speak to generally know nothing about technical support; they're told to read through a fixed support script. No independent thinking on their part is involved or allowed. Sometimes when you get to the end of the support script, they bounce you to someone who actually knows something about tech support. More often, they just tell you to use the system restore disk, zapping your configuration.
2) Their laptops are not as durable as some competitors, and the materials are typically lower quality. However, their price is also typically quite a bit less than an equivalent ThinkPad, for example, so really, this isn't that much of a knock against Dell. They aim for a mass-market, price-conscious consumer and do well at that.
3) Look into the stories about how they handle bad pixels on some of their LCD screens. There are guys on the Dell support forum who've returned LCD monitors five or more times, never getting one that quite satisfies them. Again, to be fair, all manufacturers are prone to bad pixels occasionally, but it boggles my mind that Dell doesn't just ship a cherry-picked perfect display once a customer has gotten to three returns or so. It makes you wonder if Dell hopes the customer will give up first.
4) Dell continuously shops the world for the lowest priced suppliers and frequently switches component suppliers, so a machine that your friend bought three months ago and has proven to be super reliable may not be the same machine you would get if you ordered today, even though it has the same model number.
5) Dell has been pushing "special deals" and "coupon codes" more and more frequently. It's not uncommon to get $500-700 off a computer if you have the right coupon code. However, there has been some jacking up of their "standard" prices, and if you're not savvy enough to find those deals, it has become very easy to pay too much for a Dell. Imagine how bad you'd feel if you bought the same computer as your friend, on the same day, but he had a $700 off coupon code and you paid full price. Also, their "standard" prices are not as competitive relative to some of the other PC vendors as they used to be before Dell really pushed hard on the deals.
6) Dell gives you different prices for *identical* items depending on whether you select "corporate", "small business", or "consumer" on the first page of their website. Check it yourself. Usually the "small business" prices are the best. Many people feel this differential pricing is dishonest. Others feel it's fine.
Really, though, Dell isn't so bad, and it's hard to argue against them. They're the Wal-Mart of the PC world. Decent stuff, almost always the best price if you have the right coupon codes or hit the right promotions, but for the most part, not premium quality. That's okay, since you're not paying premium prices.