A great app to collect more email signups for your email marketing campaigns is Arena! With it you can run viral giveaways on social media and collect a really large email list that you can send promotions to
I have two of my brands using Omnisend. As I understand on a feature parity Omnisend is the clear winner (especially having 24-7 support all year round) great for both your clients and your team.
I would like to understand your rationale for moving away from Omnisend, what are the stumbling blocks that your client has? Also, are you registered as a partner with Omnisend, as you would have a direct PM that would support you and your clients all the way!
It would be great to get a better picture from you and your client.
Do let me know if there's anything I can assist with.
I understand that they now have an Omnisend academy that could help them (and your team) out a lot: https://omnisend.mykajabi.com/offers/jtnXoa3n/checkout