I’m not aware of any in-market tests specifically comparing recurring vs. gift frequency language. I suspect the answer might not be the same with all gift frequencies, nor with all people.

It sounds like a great opportunity for you to test and find out what works for your audience. Based on the literature, here’s a couple of thoughts that might help you:

• Exceptional Occurrence. There’s some evidence that making something feel like an exceptional occurrence could increase likelihood of response to it and spending towards it. In a 2015 study, Sussman, Sharma and Alter asked some people to take part in the annual walk for Alzheimer’s while others to take part in the walk which only happens once a year. The first framing, which makes the event sound recurring, resulted in 35% response and $4.82 average gift while the second framing, the exceptional annual one, resulted in 46% response and a $7.13 average gift. Based on this, I’d be inclined to think that asking for an annual gift rather than a recurring / ongoing / sustaining might be more effective. But the same result might not be true when we’re talking about monthly gifts.
• Target Audience. The effectiveness of the terminology can vary depending on your target audience. For example, some donors may prefer the term “recurring” exactly because it suggests a regular, consistent commitment. Others may prefer “monthly” or “annual” because it provides a clear indication of how often they’ll be charged. These individual differences will be masked if you test this language with a random A/B split. You need to formulate hypotheses and segment your audience based on who people are e.g. their personality and take that in consideration when you create test groups and analyse results.

Regardless the terminology you use,

• The language should be clear and transparent about what the donor is committing to. Whether you use “ongoing,” “recurring,” “monthly,” or “annual,” it’s important that the donor understands the commitment they’re making. Ambiguity or confusion can deter potential donors.
• Emphasize the benefits of sustaining or recurring donations. Explain how such donations help support your organization’s mission and make a tangible impact over time.
• Tailoring the language to the donor’s previous giving history or preferences can make the communication more compelling.