You confuse terms here. Appendicular abscess means -PUS- which must be drained either per-cutaneously or operatively.
What you mean is an appendicular MASS or PHLEGMON- representing a well walled-off appendicitis ,presenting usually late, and responding in 90% to non operative antibiotic therapy.
Now which antibiotics?
Does not matter as long as you cover E.Coli and Bacterioides fragilis.
Does not matter as long as you achieve therapeutic serum levels.
A pure guess in the absence of any prospective studies.
An example of logical regimen (there are numerous to chose from)
Start with cipro and flagyl IV. After 2-3 days- when mass becomes non-tender and patient feels well switch to cipro and flagyl orally- and send home.
Do understand- whenever you see any paper advoating a certain type of regimen as "better"- the authors were/are supported by the drug company.