I have finalized my 2017 tax return with all information EXCEPT (for the first year) one my K1 is only an estimate (the final K1 will be provided to me later this summer).
So, I suppose it is better to file for extension, rather than file my return based on the estimate by the April 17 deadline and if final K1 turns out to be different from the estimate amend my return later... correct?
Yes, you shouldn't file a tax return that you know may likely contain incorrect or incomplete information. (Also, amending a return can be cumbersome and takes a while to process, so its best to avoid them whenever possible.)
For now, you can file an extension and be sure to include payment for any taxes due as an extension only gives you an extension of time to file - not to actually pay taxes due. If you have the rest of your return completed already, the estimated K-1 should be good enough to give you a confident estimate of taxes due, if any, so you can include the payment with your extension and then reconcile when you actually file your return.