Setting aside the question of level limits, because 1E hits its "sweet spot" generally before the maximum of 15th level for a druid is reached, here are several chief differences between the cleric and the druid:
- Clerics have the ability to turn undead
- Clerics require fewer x.p. to advance in level early on, then advance more slowly
- Clerics have access to better armor
- Clerics and druids have different spell lists
- Druids have woodcraft
- Druids have immunity to woodland charm
- Druids have the ability to shape change
I submit that the clerical turn undead ability is particularly powerful, especially since it is an "at will" power, and the tables upon which it relies grant a fairly powerful application of it fairly early on (a first level cleric has a 50-50 chance of turning a skeleton, while a 4th level cleric has a 100% chance of doing so, while a 1st level cleric actually has a chance to turn a shadow, which is a fairly powerful undead creature).
Given these factors, we can arrive at a fairly general formula when it comes to the creation of specialty priests:
- Turning undead is on par with shape change, although slightly above it because of the limited number of times shape change can be used.
- Better armor and fewer x.p. is worth the same as two specialized skills, with a little more thrown in to offset the druids' major power being usable only three times per day.
From this, we can turn to the creation of specialty priests. If they have an ability that is roughly equivalent to being able to defeat specific types of level-appropriately dangerous adversaries 50% of the time, and quicker level progression, they're about the same as clerics.
If they have a selections of abilities that are roughly equivalent to two specialized skills plus one generalized skill that is limited by daily use, they're about the same as druids.
That is the very rough formula I used with my demonolater class; the druid was my guide. Although one specific sub-type, who has Orcus as his patron, is on par with the cleric, because they also have the turn undead ability (along with a very specialized minor power or two, to offset the fact that they use the same level progression table as a druid).
I don't claim to have a hard-and-fast formula, but looking at the two types of "deconstructed" cleric classes in this way gives a much better guide to what is, and what is not, over- or under-powered when it comes to powers and abilities.