Welcome to FT josefarrugia!

I would book the most affordable flight.

I believe that unless you actually fly frequently, or can take advantage of a special promo, earning butt-in seats miles has a low return on investment. I am in this same boat. Butt-in seat miles are a bonus of flying and should not dictate you breaking your budget to earn them.

I am in a similar situation to you, no where close to Frequent Flier but I do have CC's. I would recommend you earn miles through your CC, whenever possible charge it and make sure you can pay off the balance. For infrequent flyers this is the best way to earn mileage.

Also make sure to register for all frequent flyer programs you may be buying fares with and diversify your holdings.

Fly for the trip not for the miles!