What would be better if need to migrate from Oracle Forms 10g , Either Apex or ADF
If Apex then Why and Why not ADF
If Adf then Why and Why not Apex
Obviously, this will be an opinion-based answer.
They say "Forms is dead" for at least two decades now, but - Forms is still alive. Guys in India still develop applications in 6i, I'm not sure they switched to web yet (how do I know? By reading questions on different Oracle forums).
ADF looked like a great choice for everyone who wanted to abandon Forms. ADF looks pretty. ADF is Java. I don't like Java (but that's irrelevant). ADF is not a simple tool and people who don't speak Java can expect certain problems. You probably know how simple it is to create a master-detail form (for example, based on Scott's EMP and DEPT tables) using Forms? Matter of minutes. I don't know about ADF, but I've watched a Java developer who spent hours (literally) to do the same task (while, at the same time, another person created it in Delphi even faster than in Forms).
APEX is way simpler than ADF (and Forms, to be honest). All you need is a web-browser. No complex IAS or Web logic installation, JInitiator and stuff ... OK - embedded listener does its job if it must, but you'd better switch to ORDS. If you have fair (PL/)SQL background, creating an Apex application is matter of several hours/days, and you'll have a nice-looking application. How robust is it? Quite, as far as I can tell. What about you, as a developer? Lucky you, you came when Apex 19.x is out. In pre-dynamic actions era (3.x), you'd suffer in pain because Forms does everything with triggers - there are none in Apex. But now, now you have dynamic actions which are the closest thing to triggers so far.
If you keep the logic in the database (stored procedures, packages) and let APEX take care about nice presentation to users, you shouldn't expect too many problems. I've heard (just yesterday) that ADF developer I know does the same - keeps everything in the database and uses ADF to create a nice application. Users don't like Forms applications because they look ugly. But hey, what do they know? Forms applications - if well written - are really good, robust, reliable piece of software.
I've heard (on one of OUG conferences) that - on OpenWorld - there were zillion APEX presentations and zero ADF ones. Apex is in nowadays, ADF seems to be out.
Would I want to migrate my Forms applications to anything? No, I wouldn't. I sympathize with you.