The short answer
Two plush soothers with cry sensors, at similar prices. Ewan has ten sounds (each layered with a heartbeat), continuous or timed playback, and a battery quoted at 22 to 35 hours. Tommee Tippee's owl or panda has six sounds, three light levels and a battery quoted at up to 72 hours.
Ewan gives you more choice and has a far bigger base of owner reviews; the Tommee Tippee runs for longer between charges. Both belong outside the cot: Sweet Dreamers suggests hanging Ewan on the cot bars, while Tommee Tippee says not to leave its aids in the cot unsupervised.
My call: Ewan for most parents, for the wider choice of sounds. Pick the Tommee Tippee if charging is the thing you know you'll forget.