Pod machines that take third-party capsules

Every pod machine locks you into a capsule shape. What varies is whether anyone other than the machine's maker is allowed, or able, to make that shape. That single fact governs what every cup costs for the life of the machine and how easy the capsules are to find.

Open and closed systems behave very differently

K-Cup is open enough that dozens of roasters sell compatible capsules, which is why the price per pod varies so widely and why supermarkets carry a wall of them. Nespresso Original has been widely copied and has a large compatible market. Dolce Gusto and Tassimo are far more closed, and in practice you buy from one company. An open system means competition and falling prices; a closed one means a single supplier for as long as you own the machine.

Availability matters as much as price

A closed system is also a system whose capsules are stocked where that company chooses. That is fine until you want coffee on a Sunday and the only shop nearby carries one brand of pods that is not yours. Before buying a machine, look at what is actually on the shelf in the shops you use, not at what is available online.

Compatible is not identical

Third-party capsules for a given system vary in build as well as in coffee. A shell that is slightly the wrong stiffness can pierce badly or leak, and a machine's newer generation sometimes tightens tolerances. This is not a reason to avoid them; it is a reason to buy one box before buying ten.

The related question

If your motivation is cost rather than choice, the other route is filling capsules yourself: machines that take refillable capsules is this same catalogue asked from that angle, and it saves considerably more per cup at the price of a grinder and some practice. Nespresso Original against Vertuo is worth reading before choosing a system at all, because those two are not interchangeable despite the shared name.