Check which system you own before anything else
Nespresso sells two incompatible systems. Original capsules are the small aluminum cups with a flat top; Vertuo capsules are dome-shaped with a barcode on the rim, and the machine reads the barcode to set the pour. An Original capsule will not fit a Vertuo machine or the other way round. The CitiZ, Essenza Mini and Gran Lattissima we rank are Original; the Vertuo Plus and Vertuo Pop+ are Vertuo.
Original has a third-party market; Vertuo does not
Because the Original format is open, Lavazza, L'OR, Peet's, Café La Llave and budget brands like Bestpresso all make compatible aluminum capsules, and the good ones brew nearly as well as Nespresso's own for half the price. Vertuo's barcode system has no meaningful third-party supply, so a Vertuo owner pays Nespresso's or Starbucks' price — roughly $1.12 to $1.62 a capsule on this page against 32 to 90 cents for Original. This is the single biggest running-cost difference between the two machines, and it is worth knowing before you buy one.
Per capsule, not per box
A 50-count Starbucks Original box at about $28 and a 30-count Vertuo Intenso box at about $42 look like a $14 difference; per capsule it is 57 cents against $1.40. Every product here carries its per-capsule price in the spec table, read on the day we checked.
Aluminum capsules and the recycling program
Nespresso's own capsules and most of the compatibles here are aluminum. Nespresso runs a mail-back recycling program for its capsules in the US; third-party aluminum capsules are recyclable in principle but are not accepted in Nespresso's bags. If waste matters to you, that program is the practical difference between Nespresso-brand and compatible capsules.
Variety packs first, then the box of the one you liked
Both systems sell samplers — Nespresso's Original Variety Pack and the Vertuo Discovery box are the widest. Buy one of those first; then buy the 50- or 100-count of the blend you kept reaching for. On Vertuo that also settles the cup size, since the barcode decides whether you get an espresso or a 7.7-ounce mug.