Last year, I took 1A03 in first term and 1LT3 in second term, mainly because I heard that 1AA3 was monstrously difficult. Like you, I had originally registered in 1AA3 but switched out of it before the end of first term. I did well in 1A03 but didn't want to make my workload any heavier than it already was. I also wasn't planning to do a concentration in math, so 1LT3 was sufficient for me. In fact, I'm convinced that the content I learned in 1LT3 is more applicable to what I'm going into than what 1AA3 offered.
I remember hearing that the first midterm for 1AA3 last year had a very low average while the exam was very easy, possibly to make up for the difficulty of the midterm. Regardless, I'm still very glad I took 1LT3 because I'm almost sure the workload is lighter due to easier content. 1LT3 uses a different textbook than both 1A03 and 1AA3 and the 1A stream does not share a significant similarity in material with the 1L stream. The overlap between 1LT3 and 1AA3 is extremely minimal (some integration by parts, at most). If it matters to you, the prof for 1LT3 is also kind and approachable, very clear, and doesn't have the intimidation that I feel the 1A03 and 1AA3 profs carried.
Some people mentioned the complications in going from 1A03 to 1LT3 and how you would have "missed out" on the content that most other 1LT3 students covered in first term 1LS3. I caught up with no trouble at all. In fact, there wasn't much to "catch up" with anyway since you learn derivatives in both 1LS3 and 1A03 (although much more in depth in 1A03, of course). The only term that was unfamiliar to me in 1LT3 (that was taught in 1LS3) was cobwebbing, but it wasn't ever asked on an assignment/test/exam. Whatever you need to know for 1LT3 will be taught in 1LT3. Any foundational knowledge would have been taught in more depth in 1A03 anyway.
I would only say that the transition from the 1LS3 to 1AA3 stream is difficult. I haven't done it, but 1AA3 builds upon heavily from 1A03 material whereas 1LT3 doesn't have as strong of a relationship to 1LS3. In any case, catch-up of 1L material is inherently easier than that of 1A material. If you want confirmation, compare 1LS3 tests with 1A03 tests and come back to me.
Overall, I suggest you take 1LT3 if you aren't pursuing a math major or minor, or if your field of interest does not suggest or demand the knowledge required in 1AA3. Or, if you like a challenge, then 1AA3 may be a good option. Everything else being equal, I don't know for certain if I would have gotten the same or a lower mark in 1AA3 as I did in 1LT3, but I am glad I chose 1LT3 because the lower workload, absence of required assignments, easier tests, and my higher interest in 1LT3 material made it less of a stressful course.
Have a look at the content on the course websites for
1LT3 and
1AA3. That may help with your decision!