In the last few months, several relatively new airfare search sites have begun fiercely competing for your attention.

The newest entrants, both so-called “meta” search engines, are from Travelzoo (fly.com) and TripAdvisor (tripadvisor.com/flights). “Meta,” according to Wikipedia, means, among other things, “beyond” or “after,” and the metas do indeed go beyond standard online travel agency fare search to include the airlines’ “private stock” fares as well as searching multiple online travel agencies. There are now six major airfare meta search sites, including Kayak.com, Bing.com/travel, Momondo.com, and Dohop.com (I'm not including Sidestep.com, now owned by Kayak and essentially the same thing). But how do they compare to the older online travel agencies (OTA's) such as Travelocity, Orbitz, Cheap Tickets, and Expedia? (Disclosure: airfarewatchdog.com is owned by Expedia, which also owns TripAdvisor.)

Advantage: Meta

One thing that meta search engines do especially well is finding airfares that the airlines sell only on their own sites. For example, Hawaiian Airlines often has special sales that can only be booked at hawaiianair.com; Cathay Pacific usually sells its New York to Vancouver route for less on its own site than it does through online travel agents. An OTA will not find these airline-site-only fares; in many but not all cases, the metas will. (That said, neither meta nor OTA will hunt down the increasing number of promo code or “twitter” fares that are only bookable on the airlines’ sites).

Also keep in mind that no meta includes fares on Southwest Airlines, which is a pretty big gap. Momondo.com makes an attempt to do so, but it’s not a very good implementation, since, in our tests, the fare results are not real time; Bing merely reminds you to check Southwest if it flies a route you’ve searched, but doesn’t itself return fare results—still better than nothing. (Southwest sends out cease and desist letters when meta searchers attempt to mechanically “crawl” its fares; perhaps Momondo gets away with it because it’s based in Denmark, beyond the reach of Southwest’s legal beagles?).

Nor do OTA’s include Southwest’s fares, but they do accomplish feats that meta search does not.

Advantage: Online Travel Agency

One of these is packaging hotels and airfare together, sometimes, amazingly, for less than airfare alone. Many times airfarewatchdog.com researchers have found a “TotalTrip” option on Travelocity.com, for example, including airfare and several nights hotel, at nearly the same price, or even less, than the airfare for the same destination. These package options are automatically presented, when available, whether or not you thought to ask for them.

Another OTA plus: sometimes your cheapest flight will be outbound on one airline and returning on a second carrier. Delta.com is not going to tell you this, obviously preferring to show you flights only on Delta. An OTA will, which explains why many times even the metas send you only to online travel agencies rather than to airline sites directly.

And metas are not very good at flexible search. Sure, you can perhaps search one to three days in either direction of your primary dates, but that’s about it. What if fares to Cancun are hundreds less in February than in March? A meta will make it much harder for you to discover this than an OTA will, since OTA’s have much more robust flexible date searches. Travelocity will search over as many as 330 days at a glance, and Orbitz up to 30 days at a time.

So will the newer metas find you a lower fare than the older ones? Is one of them a category killer? Are they better than online travel agencies? It’s hard to answer in the affirmative to any of those questions. But those might be the wrong questions to begin with.

Most of the metas and OTA’s offer free alerts when an airfare goes down in price, and that might be their most useful feature. Rather than searching for a fare yourself, you may save money by setting alerts well ahead of your travel dates and pouncing when a trip suddenly becomes cheaper or reaches a price you specify.