Yeah I'm not a fan of questions like this, and I've seen a lot of them pop up lately.
It's painfully obvious for any investor in anything that narrowing your choices to a Black or White decision is a bad idea.
Invest in LOTR, or Monster Fighters? The Answer is Yes.
Sell on Ebay or Bricklink? The answer is Yes.
Quick flip or long term invest? The answer is Yes.
Small, Medium or Large? The answer is Yes.
Diversifying everything you do, selling strategies, set choice, size choice, sourcing, etc will protect you from an "all eggs in one basket" problem. I build my portfolio to cover a little of everything, small quick flips, long term investments, small easily mailable polybags, parts to part out on Bricklink, etc. I've even started selling custom mobile frame MOCs I do myself.
The only universal advice I can give that should benefit anyone, is to keep your entry price point as low as possible, and on a related note, always be looking for the deal. Online, at retailers, or locally. Always. Be. On. The. Hunt.
There's boats setting sail everyday. Blink and you'll miss the next one.