Both can serve well, but I lean towards the less uber, yet more resourceful druid.
Adding a druid gives you an animal companion to help on the front line and a druid to add spell and melee support.
In melee intensive battles, the party now has 4 front liners (fighter, rogue, animal companion, druid) and 1 spellcaster. Heck, with their summoning ailities on the fly, the druid can provide a massive front line army on very short notice.
In spell casting intensive situations, the PCs now have 2 ranged spellcasters (druid and wizard). Once that druid hits 7th level and gets access to flamestrike, he can serve the party almost as well as the wizard in the spell hurling department.
Although clerics are strong in their way, their benefits tend to be a bit redundant at higher levels. Shield of faith is nice until you get your hands on rings of protection. Prayer is nice until you get your hands on a lucky magic item that grants the luck bonus. I'm not saying that these types of spell are completely useless at higher levels. I'm just saying that they lose a lot of utility. The bonuses from the druid tend to hold onto their value a while longer. Barkskin is a real winner for most druids throughout their entire career.
With regards to healing, it is more difficult for a druid, but not excessively so. The best way to see to healing for a party like this is via wands and staffs. When you have a lot of time between battles, it is fairly easy to use a CLW wand to slowly heal up the party. It may cost a few coins, but not a significant amount ... During battle, a scroll or staff of the approprate spell can get the job done. The cleric is better at this type of thing, but a druid can definitely get the job done well.