The availability of baseline emission data across various livestock systems is an essential component of identifying suitable mitigation strategies for the future. However, the comparative CH4 emissions of different livestock species, particularly those that represent key small ruminants globally (sheep (Ovis aries) and goats (Capra hircus)), have not been explored in the context of diet digestibility and growth potential of the animal. This study aimed to generate baseline data through comparing weaned lambs and goat kids in relation to (1) growth performance, (2) enteric methane emissions, (3) and diet in vivo digestibility of a common medium-digestibility diet that reflects the pasture type that the animals might be expected to graze. Ten castrated lambs (Welsh Mountain breed) and ten goat kids (Cashmere × Boer crossbreed) of an equivalent age postweaning (c. 12 months) (avg. starting weight of 15 kg ± 0.86, and 26 kg ± 1.02, respectively) were offered a grass-nuts diet ad libitum whilst housed in individual pens. Dry matter intake (DMI) of feed was monitored daily throughout an 8-week growth stage, whilst liveweight was assessed weekly. Enteric CH4 emissions of each animal were recorded for 72 h in methane collection chambers. We found significantly higher liveweight gains for the sheep compared to goats (163 and 84 g/day for lambs and kids respectively; SED = 12.8; P < 0.001), as well as higher DMI (1299.8 vs 881.8 g/day; SED = 64.7; P < 0.001). DM intake was comparable between species on a metabolic liveweight (W0.75) basis. The sheep also produced higher daily CH4 emissions (26.3 g/day, and 19.4 g/day, respectively; SED = 3.02; P < 0.01) than goats. However, no effect of species was observed on CH4 emissions per kg W0.75 or per kg DMI. The proportion of methane energy excreted expressed as a proportion of diet gross energy (Ym) was similar for both species (4.9 and 4.6% CH4 E/gross energy intake for lambs and kids, respectively; SED = 0.46), as were apparent whole-tract in vivo DM digestibility, although the sheep had a better feed conversion ratio (FCR) than the goats (6.6 g DMI/ g W0.75 gain and 11.5 g DMI/g W0.75 gain, respectively; SED = 1.58; P < 0.05). Here, we found that sheep were more efficient than goats when offered an identical diet of grass nuts, giving a higher weight gain than the goats (resulting in a lower FCR), whilst also producing similar CH4 emissions to the goats on a metabolic LW basis.