It is absolutely within the scope of a UX practitioner's skill set to collect qualitative (i.e., subjective) data. As to if it is in the contract's scope, with your client, is different.
In my mind, how can we make an informed decision without loading a prototype onto both devices and performing an A/B test.
The question is unclear as to why this isn't an option. But being able to do so it certainly important for performing many qualitative type tests, but not required by all.
Side bar: Note that putting a new application on two different tablets is not an "A/B Test". An A/B Test requires a "control" and a "treatment". Since the user is unfamiliar with using the application on either tablet size, you have no "control". A/B Tests also produce Quantitative (i.e., not subjective) data.
Above shows many different types of user research that can be performed which fall all around the spectrum. (Image taken from: When to Use Which User-Experience Research Methods).
The green dots represent research methods that do require the actual product to be used. But there are many methods that can be used to gather data which don't necessarily require the product to be in the user's hand.
From the list above, some examples of how to gain quantitative feedback on the differences between an app on two different tablet sizes could include:
- Focus Groups
- Concept Testing
- Desirability Studies
Getting the product into the hands of the user should absolutely be a goal though. If the customer doesn't want that, for some reason, it should become your job to convince them to change their mind. If they're asking for qualitative data, getting users to use the product is a great way to get it.
It does not require the purchase of 20 of each device. Having 1 of each is certainly a key point though. You're testing basic device size, not the device itself. While each device of the same size might be a little different, you're goal should be to present a subjective summary of a "generic 10-inch" and "generic 8-inch" device.
The client should be involved in selecting what the "generic" device is, and pay for it.
In addition to the above examples of user research methods, a breakdown of many other qualitative methods can be found in this article: Qualitative UX Research. Some examples that, again, don't necessarily (though it still helps) require you to have the product in the user's hands:
- Expert Review
- Task Analysis
- Persona Development
Running through a heuristics evaluation on the interface falls inline with an "expert review", among other methods, which gives you insight into potential issues.
Conducting a task analysis can also give you insight into if the user can accomplish their task with a 10" vs. 8" device. Maybe all your users have pockets that can only ever fit an 8" device, and a key element of their task flow is to carry the thing around all day (which they would likely prefer to do in their pocket, vs. their hand).
Persona development gives you the ethnographic landscape of the user base. Helping you to show that 10" displays aren't "hip" and "cool" or that 8" displays are the ones that are "so last year".
It is unclear from the question why you are "reluctant to [perform qualitative user research]" to gain a subjective view from your user base. As "Strengths and Weaknesses of Quantitative and Qualitative Research", from UXMatters, points out:
Qualitative research studies can provide you with details about human behavior, emotion, and personality characteristics that quantitative studies cannot match. Qualitative data includes information about user behaviors, needs, desires, routines, use cases, and a variety of other information that is essential in designing a product that will actually fit into a user’s life.
This is a vital aspect of user experience.