It really comes down to some simple questions. (Not listed in any order of importance)
Will you be leaving the mixer in one place, or needing to put it away when not in use?
The KitchenAid tilt-head mixers weigh less than the bowl-lift mixers making them easier to move.
Will you be making large batches of anything?
The KitchenAid bowl-lift mixers are generally larger and have stronger motors. The Professional 5 Plus mixers and the Professional 600 mixers also have wide-mouth bowls making mixing easier for larger batches..
Will you be making anything with stiff batters or doughs?
The KitchenAid bowl-lift mixers have a more rigid design. The PowerKnead attachment also does a fantastic job with stiff mixtures. The PowerKnead dough hook is available on the same mixers that have the wide-mouth bowls: Professional 5 Plus mixers and Professional 600 mixers.
Listen to some owners of the Kitchen Aid stand mixers:
I have had both and like both. They are both capable at the same tasks but the lift bowl mixers are stronger and the Pro 600 and 5+ have the wide bowl and that makes it easier to add ingredients.
If you don't bake much, get the tilt head. However; if you're into baking big time, I would consider the bowl lift for it's rigid design and heavy duty parts...
My experience with the lift is an intuitve fit but if you're bitten by the volume baking bug you may want a lift. If moving up to a lift, opt for the Pro600. The Pro5 is only 1/2 quart bigger than a tilt head and you will be glad you bought the wider bowl. It mixes more evenly and its much easier to add ingredients without bathing the side of the mixer.