I'd say the uke is a useful stepping stone to guitar if you're under 10 years old. Many music programs start kids on the ukulele to understand the basics and have an instrument that's easier to hold.
If you're older than that, learn guitar for guitar, and uke for uke. They're similar, but there are enough differences between the two that so long as you don't mind tormenting all of those around you with your awful beginner guitar playing, you'll progress more quickly if you focus exclusively on guitar if that's your goal. If you insist on starting with a uke, make sure you play low G instead of high G, however.