Here is a character
ECL link. It can walk you through the basics of determining how to calculate the ECL of a creature, if you scroll down, they have most of the monsters from the books in there, and their ECL, which for a Balor is +29 and for a Pit Fiend is +26
Other demons are listed like this for comparison:
Demon:
- Cerebrillith (PsiHB), +21
- Dretch, +4
- Quasit, +8
- Succubus, +14
- Bebilith, +21
- Retriever, +13
- Vrock, +19
- Hezrou, +20
- Glabrezu, +21
- Nalfeshnee, +24
- Marilith, +22
- Balor, +29
Destrachan +10
Devil:
- Imp, +8
- Osyluth, +13
- Kyton, +12
- Hellcat, +13
- Barbazu, +13
- Erinyes, +13
- Hamatula, +14
- Cornugon, +22
- Gelugon, +25
- Pit Fiend, +26
Devourer, +16
Something to note about ECL for creatures, they are always listed with their level adjustment and monster levels from hit dice added together. An example of this is the Succubus, who has a ECL of 14, but only has a level adjustment of +6, that means that 8 of her ECLs are from monster levels or hit dice, which is reflected by her feat/skills/hitdice.
If your DM really wants you to play a Balor or pit fiend, but wants to give your characters a bit of flare, you could, as an example, take the Balor at a level adjustment of +9 and give him 20 levels in a PC class. Just make sure you figure out what is part of the racial bonuses and abilities (like supernatural abilities and ability score modifiers and spell like abilities) versus the monster class abilities (like hit dice/feats/skills).
To eliminate confusion, if the DM wants it, just play the Balor as is. However, I have played a Succubus with +6 level adjustment and 10 leves in Thrall Herder and 5 psion telepath, no monster levels in the past, and compared to other classes at the same level, she was fair. Granted, she had some really powerful ability scores, a +16 to CHR is very helpful, but her lack of a spell casting level to get over the SR hurdle and lack of PC class levels evened things out in the long run.