Which is better branding an icon like nike swoosh or a brand mascot like Geico?

I believe an icon is better because it will last a lot longer. Meaning that mascots sometimes get forgotten. The Gieco lizard is beginning to lose it's grip because people got tired of it. And who was that company with the duck? Think back and see how many mascots you can remember compared to icons.

Union 76 changed it's revolving ball which was sort of like a mascot, to a flat icon. Many such examples can be found. I say stick to an icon.

I think you are confused. One thing is your logo and the other thing are brand mascots. A brand is configured by the following elements:
- Name
- Logo (the graphic representation of the name)
- Isotype (the graphic representation of an object, that's an icon. Example, McDonald's "M" or Nike's swoosh)
- Chromatism (your brand colors)
and Graphic Elements (the elements you use to re-inforce your brand)

So, Geico's logo isn't the lizzard. But Red Bull's logo are 2 energetic Red Bulls, but the Red Bulls is the brand isotype. Ronald McDonald is not McDonald's logo, it's a graphic element.

I think that to figure that out you need to hire a professional who can help you to figure this out. Send me an email if you need more help on this. I work in the Brand & Creative department of one of the top 30 brand of the world.

Look at McDonald's for instance. Every little kid knows the "Golden Arches." They may not be old enough to read, but they know that sign stands for quality food in a fun atmosphere.

It's the same with any institution. Sure the reason Geico uses a Geko is because the name familiarization. You see a lizzard speaking with a british accent and you automatically know it's Geico.

You have to find something that is not only eye catching, but something that people will remember your company by.

Marketing your business can be done either by catchy phrases like DLP It's the mirrors. or Wendys with their guys in the Wendy's hair wig.

What did Taco Bell use? A chihuahua with a sense of humor. (Taco Bell really messed up when they canned the dog) You Querro Taco Bell was etched in the minds of millions.

A decent logo. The swoosh has more impact, the lizard is a gimmick an somewhat lame. Mascots, unless incorporated into the logo, tend to fade away as tastes/the market changes. Go with a solid logo design or logotype.

People remember and then dismiss a mascot - the cycle is pretty quick.

I think it depends on the image your company would like to project.

The Nike logo is elegant and simple, and thus lends itself to under-stated placement. This is important, because they place it on all of their products, and people might not buy the products if they had a gaudy logo.

The Geiko lizard is all about putting a humorous, friendly face on an insurance company. However, I personally felt less certain about Geiko because of the lizard; it made me think they might not be a safe company because they were being frivolous. This seems to have worked with them, and scored a hit with younger people (was that their target audience?)

If you can afford it, hire a marketing company to do the testing for you so you hit your market. If you can't, at least do some informal testing before you decide.

Good luck.

They're different functions. The Nike swoosh, for example, is spare and goes on the product, in the ads, etc. and the public knows what it is linked to. When Nike does ads, the swoosh is there BUT they get BUZZ from things like having Michael Jordan endorse their brand. That he also does Hanes and has done McDonald's (the famous commercial with Byrd) doesn't hurt them--because of his greatness in basketball the tie-in works. In another decade, however, only real fans of the sport will really know who Michael Jordan is. (Trust me I had a talk with a guy who is good at baseball and wants to play major league. I said I don't follow it but really respected people like Lou Gehrig. He said, Who? Come on, the "Iron Man" whose record was brought up time and again for most consecutive games.)

Anyway, if you have a logo you can use that forever or update it some (see Betty Crocker). With things like Geico's gecko, you get a campaign. The public likes it, but you will notice Geico has other campaigns and the gecko is NOT in those (the thing with using celebs is probably getting more airplay now than the gecko). Geico doesn't have a gecko on its letterhead or such that I've noticed.
So what if you pick something for an ad campaign and it bombs? Ouch. Get a simple graphic logo and you can run with that for decades.
You can have both--like Nike: logo on everything and you run campaigns with the right folks.