Telephoto Lens Advice for Canon

I use a Canon 600D and currently have three lenses:

  • EFS 18-55mm
  • A very old 50mm Pentax I use with adapter
  • A very old (1970s?) Auto Super-Paragon 200mm f/3.3 I use with an adapter

I want to get more into wildlife photography and have started exploring bird hides. I use the 200mm I have but it doesn’t really have the focal length I’m after and obviously doesn’t have IS and autofocus.

I think I’ve narrowed down the choice to the following two but I’m very open to suggestions:

  • Canon EF 70-300mm f/4-5.6 IS II USM – new
  • Canon EF 100-400mm f/4.5-5.6L IS USM Lens – second hand (note this is the original not the update)

A second hand 100-400 is a bit more expensive than a new 70-300. I’m leaning toward the 70-300 because:

  • the max 300mm focal length equates to 480mm given it’s an APS-C sensor camera, which should be more than enough and significantly more than my current 200 (equivalent to 320mm)

  • the push-pull feature on the 100-400 I’ve heard has issues sucking dust and moisture in

  • it’s cheaper than the second-hand 100-400

  • it’s generally better to have a new lens if possible than risk issues with second-hand

However, I’m aware that the L series is superior and has weather sealing. I can no way afford the 100-400 IS II, even second hand, so it would have to be the original second hand. But I’m not sure it’s worth it given I’m still very much an amateur.

Is there anything I’m missing or is 70-300 probably sufficient? Are there any others to consider? Any advice appreciated!