it's simple, your servants for the battle tower are as follows.
Hermosa = Weapon Skills coest less to improve
Kenneth = Magic skills cost less to improve
Igor = Better Magic based creature
Jonaleth = Better Weapon skill based creature
Wesson = Armour Enchanter
Radcliff = weapon enchanter
Allan = adds the option to make his allen brew
Barbatos = adds the option of making multiple potions that increase your resitance
the best ones to choose are simple if you want to get the most out of the game exp wise.
Trainer = Hermosa
Because it will allow you to use the book of war to increase your skills. If you choose Kennith you will have to give him the book to upgrade the battle tower so choosing Hermosa is the better option because it will allow you to get the skill points from the book as well as gaining the exp for her upgrade quest.
Necromancer = Jonoleth (because he's funny)
Have'nt noticed any real benafits either way but I usually go for Jonaleth because he's so bloody funny especially in the FOV section of the game, Igor freaks me out lol I think you may get an extra skill book from Jonaleth as well but I'm not 100% sure it's been awhile since I chose Igor.
Enchanter = Wesson
Radcliff only has 1 upgrade quest while Wesson has 2 you will not only miss out on some extra exp for choosing Radcliff but also a skill book.
Alchemist = Barbatos
If you choose barbatos then you will gain access as I said to potions that increase your resistance. You will also have the option of using the potion of wisdom to increase your stats. If you choose Allen then you will have to give him the potion of wisdom to upgrade the battle tower.