There is no limit on invoices of more or less than 1000$
The limit is yearly and you should consult an accountant for this.
The difference is not between Invoice E or C but between being "monotributista" or "responsable inscripto.
I understand (not being an accountant) that these are the boundaries for being monotributista
I think we could only be up to Category H
That means that we should not earn more than 896043,90 $ in a year (and depending on that value there are several categories inside monotributista)
That's the difference between monotributista (cheap and almost no administrative effort) or responsable inscripto (21 % of each invoice / less a % of what you spend / more a lot of other presentations and taxes). Monotributista is only (Today) up to 896043.90 pesos argentinos yearly. More than that it should be only "responsable inscripto"
As Julio states invoce C or E depends on your customer.
Monotributista -> Argentine customer in Argentina : Invoice C (it pays IIBB - Ingresos Brutos)
Monotributista -> Foreign Customer in another country : Invoice E (Export Services does not pay IIBB)
I expect this is clear for you.
About which is the best or cheap I prefer not to mention because I think it depends in confidence on the Provider, where is the office, how many invoices and which value, and how time consuming is each option. I didn't take time to make that evaluation.
What I mention is about personal experience (making mistakes in the process) and I am not professional in that.