What's better? By big-bang I mean an all out blitz of press releases and marketing coverage in order to generate a lot of buzz for their startup. Ramp-up, obviously is the opposite, usually means focusing on a core group or niche first, then expanding their target reach from there.
I sense a lot of startups prefer big-bang. The trend has typically been to come out of stealth and suddenly flood TechCrunch and others with press releases. They receive a huge bump of traffic, but more times than not, traffic flattens then slowly tapers off.
In addition to stating your preference please give some examples of how you would implement such a strategy, and/or a startup that has successfully implemented one of the two.
I had my startup in mind when I asked the question. But the same advice can be applied to any consumer-focused web startup, possibly targeting a niche.
Also, here as some examples of prominent startups for big-bang and ramp-up:
Big-Bang: Twitter, YouTube
Ramp-Up: Facebook, Google
It depends on the type of business, as well as other variables.
Stealth mode is needed in many cases for the time you spend developing your product, in order to get ahead of potential competition. If you have a good business model, others will try to copy you, so you want to develop your advantage first.
Another point worth considering is that different online tools rate how popular things are, which means that in order to get noticed, you need to achieve critical mass very fast. If you want to be noticed on twitter, Digg or many other sites, big bang is almost your only choice. You could spend months building yourself up in such a community, but having a bang means instant mass exposure. If you want to make a "top something of the week" list, you need a very good week, rather then an above average month
If you are not in a hype related business, and do not need economy of scale advantage or network effect, it might make sense to grow slowly, developing a community of loyal follower, and growing over time. It also lets you improve your product along the way, which means that when more customers show up, you will have a much better product.
Be sure to plan it all ahead, since you don't want to burn out before you achieve your targets.
What kind of business are you looking to promote?
I would honestly ask yourself how buzz worthy your product is.
99 out of 100 startups and business domains wouldn't be of interest to TechCrunch, Digg, Twitter, Mainstream Media, even if you arrive out of nowhere with a great product.
Even moderately successful blogs and specialist media would want some combination of amazing product + interesting backstory + brilliant execution + customer traction before writing about you.
Thus, stealth mode could end up being a wasted opportunity when you could really be interacting with your communities and trying to fire up real grass-roots word of mouth marketing.
In addition to Jasons point re finances.... Slow burn also allows you to tailor and feedback into your marketing and PR efforts as you learn what works and what captures attention. Don't burn your yearly marketing budget on a product launch that may dissapoint!
How much money do you want to spend?
As you say, a single bang dissipates fast; usually that's not enough to get critical mass. If you're not building on the shoulders of happy customers, you have to force your message into people's eyeballs, and that means money.
Growing slow and strong can be done without money, but can take years longer. Depending on market conditions, that might be OK or it might be fatal.
Neither of these ways is "better" in an absolute sense.
Benjamin's points are good ones along with other comments above. Some thoughts:
Don't believe your own hype
Most of us "eat our own dog food" as the cliche goes a bit too much and think because we believe our product is wonderful and incredible and magical that everyone else (media, businesses, consumers) will feel the same way. Often doesn't happen that way. Having proofpoints (especially happy customers, case studies, success stories) is a key element of getting people to think it's wonderful, incredible and the rest. Plus often it takes time to get the product "closer to right" than when you launch. Real customers, real feedback, real product enhancements. That will ultimately help a lot.
The product big bang
A huge part of the answer is your product and the potential it has to create the big bang with no marketing. That is true for a lot of the big web properties. Some seemingly explode like Twitter but it was because it delivered a service that nobody else did. It wasn't because they spent a bunch of money on marketing, the product caused the 'big bang'. Likewise with YouTube. Even though there was already online video there was something that they offered that the others didn't which caused it to explode. It hit a spot of need, even in spite of competitors out there, and exploded quickly.
Cost of customer acquisition
Ultimately that's what it's all about. How much does it cost to acquire a customer via SEM, ad banners, partnerships, affiliate marketing, PR, however you market the product? Then how much can each of those scale? So SEM may be my lowest cost per customer acquisition (hah) but it may not be able to get the scale in numbers you want.
And to the point of fast vs. slow, typically with new ventures it's going to take time until you really know the acquisition costs. And those should improve over time as you tune your marketing messages to improve the traffic they drive, strengthen your credibility which helps effectiveness of your efforts, tune your site to improve conversions, tune your tactics to improve your overall acquisition costs. At the same time you have to be opportunistic.
All the best,