Blues - Require more care, pristine soft water - nitrates under 15 or so, warm - 84 degree temps, weekly 50% water changes, get 3 inches, don't live long, most come with lots of disease issues, can be quite feisty. Are very colorful. Like pH's of 6. - 7, however if the water is SOFT they do just fine in 7.4 pH.
Bolivians - 4 inches, live longer, are hardier, can do fine in 80 degree water, can take a little harder water then blues - but still on the soft side, nitrates - can take a higher level - say 20ish for normal living. Can survive some of your mistakes, can get fiesty - but usually not as fiesty as the blues. (imo). Are rather drab gray unless spawning, or facing territorial challenges. Have bigger dorsal fins, longer flowing caudal fins- with beatiful red extensions. Golden bellies when spawning with light silvery skin. They also have bigger eyes and faces than the blues. pH's of 7. - 7.4 preferred, can do well in 7.6.
Let your water choose which one, if you have water on the harder side, best go with Bolivians. If you have a taste for a challenging fish from start to finish, and have soft water, blues are very pretty. You might avoid LOTS of trouble with blues if you order from a reputable breeder, and NOT buy one from your lfs. (Unless you lfs is selling local bred rams).
I breed both, have both and really like both.
Cathy
Pics - Female Bolivian - she is spawning, Male Blue - he is ready for action.