is this setup for life? if so I wouldn't do either. both species will outgrow a 20 L. if you want something to stay in the tank for life I'd go for something like rams, appistos, shellies (not CA/SA of course). if you're doing one or the other I'd go firemouths because on average the species is less aggressive and hopefully wouldn't fight amongst each other during pre and post spawning rituals.
i would get CONVICTS!!!!!!! i don't know the exact dimmentions of a 20 gallon long but convicts will only grow to 7" MAX....... usually 6" Max...... and it takes them a long time to get that big. so a pair of them would be cool IMO. firemouths are somewhat fussy about breeding and can beat the out of eachother. but if you get a male and a female con then you are almost guaranteed that they will breed.... just what i think tho.. also apistos would be cool too.
i already have 3 10g setup for Gold rams, German blue rams, and apisto cockatoos. Looking for something bigger and have heard from several sources that Cons and Firemouths can be kept as breeders in a 20L. I am thinking Cons also but it would be nice to have firemouths.
I would suggest a 20g l as the minimum for both species but it is doable and I am in fact doing it myself right now, with convicts. The footprint is the same as a 29g, only not as tall but that's alright since both species tend to be bottom/mid level dwelling species. Make sure you keep a close eye on water quality. Having aplace to hold the fry would also help a lot, I know from experiance that keeping a pair of convicts WITH growing fry in a 20g l is a PITA. As for what fish to choose, it's dfficult. Convicts are easier that's for sure but if you want a bit of a challenge try the firemouths, though they aren't very difficult either. Personally, I prefer convicts.
i say convicts as well...my firmouth that i had in a 29 was looking cramped as he grew...so i moved him to tha 55gal with some plecos...and the convict i moved to the 55 as well but i now put her back into the 29 she is doing fine in there i can see two cons in a 20L...i think they'll appriciate the space to swim length wise more then hieght(you know instead of a 20H)
I totally agree that Sajica would be alot more rewarding than firemouths or convicts. If you are going to to do firemouths or convicts, you might as well raise guppies or swordtails. If you are going to go through all the effort to get a breeding pair etc., do a species that you won't find at every petco or walmart in the US. The 20L should work, although it is a bit small, just put in a few clay pots and some driftwood and you will be set.