A couple of thoughts:
1. My experience is that bags/slings/holsters are very individual. For instance, for shooting while traveling or hiking, I use a Tamrac holster. Holds a body and 3 lens plus smaller crap (SD cards, battery, rain sleeve) just fine. But mostly I like how it fits me and the other gear I often haul when I'm hiking or just bopping around the city. I've never gotten in to sling bags but I get the people who do. I'm not wild about backpacks (unless you're using them primarily to haul stuff from one location to another) but many people swear by them. I've used a tactical vest in the past--some people love 'em and others hate 'em. The point is--you're going to want to try something on and make sure it rides well on you and ergonomically is a fit for how you work and how you'd likely use it.
2. I really doubt any sling is going to hold 2-3 bodies and 5-6 lens especially if one is a 500mm. That's backpack territory. As I said above, a backpack is great for allowing you to haul a lot of stuff from one location to another. But if you've got a backpack that is going to haul multiple bodies, lots of big glass plus all the other stuff you usually need for shooting (cards, cleaning stuff, batteries, filters, speed light or two, small notebook, maybe a laptop, some rain protection) then the way to access it is to take it off, put it down someplace, unzip the relevant pocket, assemble camera, zip back up, place backpack on (last time I did a shoot where I didn't reassemble my gear but left it out and turned my back, my manfrotto tripod developed legs and walked away). Nothing wrong with that but it's not very quick or spontaneous. May I suggest a two-fold solution: get yourself a backpack that can haul all of your gear, is padded (so gear is safe and dry), and is comfortable to carry. And continue to use your tamron bag or maybe even something smaller (small lowepro sling or a black rapid strap) so you have quick access to a body and lens for impromptu or reaction shots.
You can absolutely get a lot of good bags for $150 or under. My advice for you is:
--try it on. Make sure it fits you and is comfortable. So even if you buy something online, go to the local Best Buy or camera store and try stuff on first.
--be clear on your purpose. Is it a one-stop location for all of your gear that happens to be portable? Or is it something for you to take in to the city when you shoot? Or take day hikes with in to the mountains? Questions like this matter. It determines if you want to be able to attach a tripod (and what kind of tripod). It determines if you have a pack with a bunch of zippers and pockets (which make it easier to get gear out of quickly but also easier for pickpockets to steal stuff) vs. a pack that basically holds gear and you have to open the whole thing to get in to it. Do you want a water cover that comes with packs you expect to be outside and away from shelter? Do you want space for a laptop...and even other stuff (so it's your combination day pack and camera bag)? If you're going to stuff it under an airplane seat or an overhead bin (b/c you'll be taking long trips with it) then that has some considerations. Does it need lumbar support for long hikes or b/c you're going to keep it VERY full?
--will it fit your gear? For instance, you didn't mention a laptop but increasingly serious shooters are taking a laptop with them, transferring files as they shoot and then they can view on a screen and adjust mid-shoot. Frankly, I've known some folks who have taken a backpack or messenger bag and then added velcro inserts (Temba and Domke are among many gear manufacturers who also make generic inserts that allow you to turn a generic bag into a camera equipment carrier). BTW, this is a very under-rated approach...overseas there is a lot to be said for a backpack that doesn't say "Lowepro" or "Canon" on it (and scream "expensive camera equipment" to thieves and pickpockets).
Hello,
So I've had a tamron bag that has lasted a very long time. It's a very durable bag and I love the thing but it can hold a body and maybe 3 lenses. I recently picked up a 2nd body since I decided I want 2 camera bodies again for many reasons, plus an upgrade was nice.

While I love this bag it can't fit my stuff anymore. I want another durable bag that can fit about 3 bodies, and maybe 5 or 6 lenses. Big lenses too, my largest one now is a Sigma 50-500mm. If possible, I'd like the bag not to be insanely large but not necessarily small. I want to be have the option too sling it over my shoulder maybe or at least carry it at my side. I don't want to only be able to roll it. Hell, it doesn't have to roll at all.
Since I've never really looked into bags myself I'm not sure if what I'm asking is illogical or not. Please lead me in the right direction if I'm completely off about bags. I'm sure you guys know a lot more than I do. It'd be awesome if the bag could be under at $150. Don't wanna spend too much!
Thanks,
-Luke