A little discussion below in the photography realm.
100-400 below My wife uses both a 50D and 70D switching mostly between the older 100-400 and a newer 500mm. My thoughts are the older 100-400 in the $700 range would be a good buy if it's not been abused.
Do a search here in the Hunting and Shooting realm for "Cookie's" in the Keyword box and some of her examples will surface. Her shots will be a mix or the 100-400 and her 500mm and remember one is looking about 80% percent reduction resolution at images on this site.
With a little practice the 100-400 can be stretched out and still handheld. Not horridly heavy and it does not take up a full seat in ones rig. Definition looks great and that zoom range is about ideal for wildlife and field type sports. One can macro down pretty close with it, but depth of field is a little wanting. Still that kind of glass is not really made for doing bumble bees.
One of Cookie's friends, a fairly well equipped pro, did the Tamron 150-600 for about 6 months. For the money, he thought it was OK, but certainly not the equal of his Nikon glass. Just not quite as sharp, and a little slow in the early and late hours.
Don't know what Costco is doing now with their kits. Cookie's 70D came with something like an 18-135 and a 70-250mm. The latter, does OK, but when it's stretched to 250, one has to put in some real effort to maintain stability. The additional weight of the 100-400mm makes it much easier to manage.
Also, remember with a crop sensor one gets about a 1.6X multiplier effect. That being the 100-400 is essentially the equivalent of a 160 to 640 mm on a full frame camera.
Let us know what you decide, and start putting up some stuff.
