Which is better for a small kitchen?

The Terra. It is about 3.7 inches narrower and 1.3 inches shallower, so it fits where the FIZZpod S may not. The FIZZpod S is taller and wider but lighter.

Which one is better for a family?

The FIZZpod S, because it ships with three bottles, so you can keep chilled water carbonating while another bottle is in use. The Terra includes one bottle, so a family would likely buy extras.

Is the SodaStream Terra worth stepping up to over the FIZZpod S?

If you want a proven machine with a long owner track record and a cartridge in the box, yes. If spare bottles are your priority, the FIZZpod S gives you more of them.

Do both use the same kind of CO2?

Both are manual makers that carbonate from a gas canister. SodaStream uses its own canister system, so check that refills are easy to get near you before choosing.

Both are plastic manual units with few moving parts, so neither has a clear durability edge. The Terra's much larger owner base gives more evidence of how it holds up over time.

Can I carbonate juice or other drinks in either?

Carbonate water first, then add flavor. Carbonating sugary or pulpy liquids can foul the valve and is not what these makers are designed for.

SodaStream Terra vs Drinkpod FIZZpod Soda Maker

The SodaStream Terra is the safer buy for most people. It has a far larger review base (4.8 from 2,333 ratings versus 4.3 from 76), a smaller footprint, and a CO2 cartridge included, so it works out of the box. The FIZZpod S offers three bottles and a lighter body, which suits households that want spare bottles ready in the fridge. Both are manual plastic soda makers with gas canisters, so carbonation quality is close; the Terra wins on track record and support, the FIZZpod on bottle count.

Choose the SodaStream Terra if you want a proven, compact starter kit with a gas cartridge in the box and a strong owner rating.
Choose the Drinkpod FIZZpod S if having three bottles on hand matters more to you than a long review history.

Prices

SodaStream Terra: $39.99 at eBay (checked today)
Drinkpod FIZZpod Soda Maker: $39.99 at Amazon (checked today)
Difference: both cost the same today