Honestly, I am a fan of sand, no dirt...all the plants I keep grow great with just sand that's just me, though...you could do dirt if you like the look
Here is a picture of my 2.5 gallon tank. This is an older picture and I have since add a filter that I put behind the pink decoration and a small heater on the back wall in the middle. The plants I have are supposed to be easy to care for, one of them is an Anubias but I forget what the other plant is. Sorry! But I thought you might like to see what it could look like with the plants.
I do dirt in my 2.5 gallon. if you keep it shallow most of the plants will be stunted and remain smaller. I recommend a fine gravel cap. it will hold down plants better in a shallow soil setting. I have sand and it's a pain to get anything to stay down sometimes.
I do 100% water changes all the time and i don't have filters . How you do 100% water changes with sand? I always had rocks because it easier to clean, just rinse. How you get all debris out of the sand? Sound complicated for me.
plant suggestions.. its a long narrow tank... so he has room to swim...
i have i big clump on anacharis already ... i know i want a moss ball (my daughter want shrimp - aka betta food) and a snail of some sort for alge eating... i want to get the shrimp, plants, and snail soon and i'll add the fish in 2 weeks or so...
(and some wine for me, shrimp and snails make my skin crawl)
now if i could just find a fancy betta... hmm then i'd have 2! :)
Check out the eBook Betta Fish Care Made Easy
that's just me, though...you could do dirt if you like the look 
