If you only have 1 unconfirmed transaction, they're equal in security... The main differences are that abandontransaction is a command you can run from the cli or the debug window while your client is running and it only abandons the 1 unconfirmed transaction you enter as a parameters. -zapwallettxes is a startup parameter, it can only be issued when starting the client, and it usually takes a lot longer to do this compared to using abandontransaction. Zapwallettxes also removes ALL unconfirmed transactions you have broadcasted...
On the other hand, abandontransaction doesn't always work (you get an error message that the transaction cannot be abandoned). In this case, you'll have to use the zapwallettxes parameter (i'm to lazy to look up the spelling, but i think -zapwallettxes might be misspelled).
BTW: next time you have a problem, it might be better to start your own topic, replying to a 6 month old topic is usually considered necroposting, eventough you have a valid question (just a tip)
I am sorry, using -zapwallettxes is not the primary solution. Before trying this workaround you should restart bitcoin-qt with -walletbroadcast=0 and try again. This way it worked in my case.
Thanks for the quick response and sorry for necroposting, I didn't want to ask a question to a similar topic again.