I did the BG Fit and know others who have also... no experience on Guru.
Generally I was satisfied with BG for a first time fit, but it will be looking at something more advanced next time.
As stated, the fitter is probably the more important variable.
Talk to your local fitters, get a feel for them and the service.
how long have they been doing fits ?
(and how long on the current
platform - e.g. BG or Guru - and what if anything were they using before
(20 years fit experience on a platform prior to the Guru system obviously could have value...)
be familiar with the fit platform you are choosing and be aware of how well implemented it is at the shop:
e.g. say Retul... in the beginning they had to put leads on one side of you, and then take them off and put them on the other side. Now they don't need to do that, and in fact they have a whole adjustable trainer bike system dealers can buy into. That's not to say that ALL dealers are necessary setup to not move leads around, or have that trainer system. This is just an example to say that not all dealers working with a given system will be technically equivalent. A BG fit on some adjust-a-bike from the Stone Age may be a less desirable experience, whereas another place down the road offering BG might have a better system. They should be able to smoothly move you from position to another on-bike to find the best fit for you.
are they working with people at YOUR level of the sport? or beyond
big difference fitting your average guy on his first road bike and fitting pro athletes.
you can probably get a sense of this from the shop...
is it a pure roadie shop, or are there just road bikes between the kids bikes and the cruisers.
are you going to be fit on a platform that's next to a busy area at noon on a Saturday?
personally I prefer a private space, and/or before or after the shop is open for retail.
ask questions which may be relevant to your needs and gauge their expertise and answers
e.g. if you have a leg length discrepancy, or foot/arch issues, or whatever - have they dealt with those before, and how?
you will need have some knowledge yourself (do some reading) to determine if they are out in left field...
also see what sort of products/solutions they recommend for those issues,
my limited experience with BG Fit suggests the range of their solutions only extends to Specialized products
(saddle, insoles, etc .... I hate those friggin' insoles - they are all wrong for my feet)
It would be nice of the if the place had experience on a wide range of products and wasn't locked into a particular brand. Be open to their suggestions if you think they are not just trying to sell you product. e.g. Speedplay pedals could be appropriate solution to some problems due to their wide range of adjustments. Particular shoes could be better than whatever you might happen to be riding now (wider toe box, stiffer sole, whatever)
See what they offer in terms of follow-ups to the fit (e.g. can you come back in 1,3 weeks or months and do an adjust?)
I'm not sure what is typical in this regards, but it would be nice to know you can do the fit, see how it goes and pop in for a tune-up rather than another $200-$400 fit session.
Talk to people at your level of cycling in the area who have used those fitters if possible - were they happy? Were they trying to resolve issues similar to your own? There's no point talking to some guy who just bought a cruiser and needed the saddle set at the right height give or take an inch, vs. someone at a competitive level tried to eke out the last 5 watts.
PS. if you're interested in playing arm-chair-fit-specialist, this is a decent app for some self-analysis.http://www.ubersense.com/
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But nothing beats a proper fit.