What are you going to do with it? Are you a home baker that does small amounts? I have a KA 6Q with the lift bowl and it works well if I am doing doubles or triples of things, but it really does not work well if I am trying to do something small or for something like a single recipe of Wilton BC. I borrow my mom's 5Q with the tilt head for my classes because it is smaller and easier to carry. It usually stays at my house between classes and I tend to use it ever 3 out of 5 times over the larger mixer.
Also have you measured the space between your counters and cabinets? The lift bowls are taller and don't always fit under cabinets.
My advise is just to look over your kitchen. I live in an old townhouse and if I got the tilthead, I would have to pull it out sideways to the edge of my counter to open the top because all my counters have cabinets. This may seem obvious but believe me, Its not...I can say this from experience
I am fortunate enough to have owned both a tilt-head and bowl lift model KA mixer. They both have their pros and cons...
TILT-HEAD: The model I owned in this style was a KA classic stand mixer (the base model). I loved the tilt head because it was very easy to tilt the head up to change attachments, and the bowl locks into place making it very stable. However, the mixer motor was not as powerful as the one I now own, and I actually burned the motor up in my first mixer. (I am the only person I know who has actuallly killed a KA...maybe it's just me!)
BOWL-LIFT: I now own a Pro series 6 Qt. mixer with a 525 watt motor. I absolutely adore it, having had it for about 4 years now, using it at least 5-6 times per week. I make all my batters and meringue based buttercreams, plus I also use it to make MMF with no problems! Still runs like brand new. However, I have the new KA 11-wire whip that I use for making meringue, and I find myself having to take the bowl completely off the mixer to remove the whip attachment and change to the paddle beater (you can easily place and remove the paddle beater with the bowl in place...only have this problem with the wire whip.) I also find myself making larger batches most of the time. I never make smaller than a double batch of icing, and the bowl will actually handle up to 3 mixes worth of cake batter.
The bottom line is you will love the mixer, no matter how the bowl attaches and whether or not you have the tilt head. I would base my purchase decision solely on the motor wattage, especially if price is not an issue.
I have both the K6 and today just got a tilt Artisan, my ultimate luxury kitchen addition...two mixers!
I have had the 6 for about 5 years and LOVE LOVE it. I agree about small batches--it's not as good for that, but I just mix small stuff by hand. until now.... 
I got the Artisan so I can mix small stuff at the same time, AND--drum roll---it's the only tilt head that can take the 5, the 4.5 and the 3-qt bowls, which makes it worth it's weight in gold for multitasking.
But the 6--there are some things you just can't do with out it. Like Sylvia Weinstock's big IMBC recipe--it will fill that 6-qt to the TOP every time. No way that will fit in a 4.5...
just my two cents.
If you have to choose, get the 6. You won't regret it. It's the difference between a 9-cup bowl and a 14-cup bowl... 
I am fortunate enough to have both the bowl-lift Professional and the tilt-head Artisan. I definitely prefer the tilt-head mixer and use it 9 out of 10 times. My dream-come-true KA mixer would be a tilt-head mixer with the super powerful motor of a bowl-lift mixer. Why can't they make one of those?!
I have them both, I use my 6qt. for heavier things cause it has a large motor. I use the 5qt. to make frosting in. I just wish they made the Pro 6qt. in a tilt head because I find that easier.
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