Knowing the type of business your in would help.
Some types of businesses will lean heavier on one or the other...some really don't require much of either...more just expertise, honesty and simply the ability to communicate effectively. Some good input above already.
In my humble/personal opinion, based of 20+ years of direct customer service/sales, screening countless outside sales reps and everyone under the sun trying to sell me or the shop owners the latest and greatest everday...I think both of these things are highly over rated...unless sales/marketing are the core of your business itself.
I may be an anomaly and highly biased though as I find myself pretty well disgusted and disenfranchised with every form of mainstream marketing and targeted marketing that gets blasted out on email, print, social media, radio, TV etc. The main reason I pay for add free music/streaming services...the ridiculousness of 95% it genuinely just makes me angry anymore.
If you are good at what you do, make, manufacture, service you provide etc...and there is a genuine demand/need for said item or service...word of mouth, your quality, reliability and reputation will do the heavy lifting. Local sponsorships, community involvement, smart hiring and keeping a tight leash on your reputation and quality control will take you farther than any add campaign or sales course starting from ground level.
If your in a niche market or have a much smaller customer target base, that can changes things a bit, but the core concept still applies.
Another thing to consider as you look to grow is the mindset of sustainable growth. Growth and the ambition for it are great, in moderation. Growth that out paces your ability to maintain quality, consistency, reliability and a healthy work environment for you and your employees, among many other factors, is more often than not unsustainable and will result in negative impacts to your business short and long term.
This is all very general stuff, some specifics on your field of focus would definitely help folks offer some more tailored input.
Some concepts can be broadly applied, many will be much more specific to your individual business.