Hello @warrior-princess!
If I were in your shoes, and had nothing in my telephoto slot… (I have an 18-200 kit lens sitting there right now)… I think I would go for the telephoto. My reasoning is that for anything that involves people at all (events, video, portraits) you’ll be reaching for that telephoto far, far more than you will an ultra-wide. Which one you pick depends a lot on your budget, but if you’re relatively new, experiment with something cheaper. You can always decide to upgrade later, and it may turn out that you find yourself not using it very much.
Here’s some recommendations as to which one you could get:
https://www.the-digital-picture.com/Canon-Lenses/Canon-Telephoto-Lens.aspx
Don’t feel like you need to go for the one with the widest aperture or all the bells and whistles, just stick to your budget. If I’m needing a pro-grade telephoto for a wedding or something, I’m usually renting anyway. For stuff in my backyard, I’m using my budget-grade 18-200, even though it’s technically “not as good.” (go back and read what Lydia said…) You can always upgrade if you find yourself reaching for that telephoto a lot but you won’t even know what features you want until you’ve been playing around with it for a while.
To fill in your low-light capabilities and if you’ve got some money left over, I’d then recommend looking into a prime lens. You can get a feel for what focal length you really like. The 50mm is a good choice (very versatile lens! You can use it for headshots, full body, landscape, etc… and it’s also very inexpensive at around $80 on the used market at KEH.com). Other popular options include the 35mm or the 85mm. If you have a zoom lens, you can “practice” being stuck in a particular length by taping the zoom ring to get an idea of what you’d like to pick up.
If you play your cards right and shop used (I’d recommend sites like KEH.com or bhphotovideo.com), you could pick up a budget telephoto and a 50mm 1.8 for less than $300. These will keep you busy for a good long time.
And then after a couple of months, if you’re still wanting something on the wide end, you could pick up a 10-18mm used for around $150.
Hope this helps (and let us know if/when you land on something!).
—Logan