I have the tilt head Ultra power model which we bought used for $70. It works very well and has done everything we've asked of it. No problems with power for kneading bread or mixing heavy cookie batters. However, there are several reasons why my next one will be one of the Pro Line bowl lift models.
1. The bowl has gotten stuck in it's socket a couple times, requiring a firm whack on the handle to dislodge it. This was due to kneading heavy bread dough for long periods and the bowl continually tightening in the socket. Eventually the screws that hold the socket stripped and I had to replace them with rivets. The bowl lift models have a completely different way of attaching the bowl so this isn't an issue.
2. The mixer tends to "walk" on the counter when kneading bread dough. This hasn't led to a fall yet, but the potential is there. It moves about pretty well. This is primarily due to....
2A, The tilt head mechanism. It's not very rigid. The head moves quite a bit in relation to the base, and I have heard of the cast metal at the joint breaking. There is a way to tighten the joint, but I haven't been able to get it to stop wobbling.
3. The larger capacity bowl. This is both a pro and a con. Whipping smaller amounts of things, like one egg white or a half cup of cream can be difficult as the tines of the whisk don't quite reach. This may not be an issue on the newer models. The extra capacity for double or triple batches more than makes up for it though.
4. Motor noise. The motor isn't too terribly noisy on mine, but it's a conversation stopper. The DC motors of the pro line models are REALLY quiet.
5. The spiral dough hook. This is a deal breaker for me. I won't ever again buy a mixer with the regular C shaped dough hook, and the spiral hooks are only available on the bowl lift models. No, you can't just buy the spiral hook, I checked into it. It won't work with the shape of the bowl on the tilt head models. The spiral hook works at least ten times better than the C hook.
Having used a pro model with the spiral hook quite a bit in a class, I honestly think there is no comparison. If you intend to bake bread, GET THE BOWL LIFT MODEL!!! Trust me, it'll make your bread baking SO MUCH EASIER!!!