You seem to be approaching this as a financial-only decision, which strikes me as a flawed decision model. You are likely to be much happier at one than the other, so they're not interchangeable.

That aside, let's look at financial implications of choosing to consult in marketing vs. web development.

As you noted, marketing is a better fit for ongoing retainers (for recurring revenue), more so than web development (which tends to be one-off projects, although approaches like Growth-Driven Design are bringing retainer models to web development).

If you choose marketing, you'll have a lower barrier to entry in finding consulting clients (because anyone can claim to be a marketer). You can probably adapt faster, too; it's easier to offer clients a new marketing channel than it is to learn a new programming language. And you'll likely build a portfolio of successes faster, which you can leverage to get paid on value.

In contrast, web development's higher barrier to entry (because you need baseline technical skills first) means less competition and the ability to charge more on a per-unit (hourly or project) basis. But longer projects and fuzzier returns mean it's going to take longer to build a salable portfolio of successes.

Consider where you can demonstrate a bigger impact. It's easier for clients to see marketing as an investment in growing their sales. That's not always the case with web development, where clients are more likely to see your service as a cost rather than an investment.

Finally, whichever way you go, invest in building your client service and strategy skills. Those will help you stand out against competitors who may be technically savvy but poor at delivering the service, or great at the tactical work but poor at creating a coherent strategy.

Good luck in the process, and glad to do a Clarity call to answer any further questions!