Any major difference between weld/bolt in subframe connectors?

The weld in type will be better over time. Bolt-in connectors will help, but they're there because the body flexes. This flexing will stress the bolts, and over time the holes will stretch.

You can weld in the "bolt in" type of connector, so if you don't have a welder you can get the bolt in type, bolt them in, and then drive the car down to a muffler shop (or similar) and have them welded in while the cars on a lift. Shouldn't take very long at all.

As far as the square tube style connectors vs the stamped kind that welds to the floor boards, you can argue back and forth for which is actually stronger. I personally think that the tubular type will add more to the stiffness of the frame, but they don't look stock. The stamped type that welds to the floor will practically look factory if finished properly, but they're much more time consuming to fit and install, as they require a lot more welding (the entire length of the connector, on both sides, twice). If your car is on a rotisserie that's not a big deal, but if your car is already complete with interior etc, the tubular type is much easier to install, as you won't have to run 20' of weld while laying on your back after removing all of your interior. And you'll have to fit them to the floor, which usually requires several trials, with lots of cutting and grinding in between.

It sounds like the bolt in kind is probably the better way to go for the tools you have available, and you can get them welded in when you get access to a welder. Or, just take it down to an exhaust shop and have it done there, with a lift and a MIG it really should only take half an hour if everything is already bolted in and cleaned up.