I don't skim on my 55g either, so you are asking the wrong person about skimming!
Yes, a 20g gives you more leeway, more room for stock, but really I did not see the difference between the 20g and the 5g as far as temp, salinity and pH.....ALL small tanks are going to need at least a daily glance for temp and water level.....
And no, there isn't much difference money wise between setting up a 10g or a 20g, but that wasn't the posters question.