Sometimes when a prosecutor has a serious case that he feels confident he can win, he will refuse to come below the top charge and make any kind of an offer on the case. That leaves a defendant only two alternatives -- get whatever the judge is willing to give you or go to trial.
A 20 year sentence
is pretty steep, and but -- assuming a conviction on both charges and consecutive time, it might be substantially worse after a conviction. There's an old saying, "Be careful what you wish for, because you just might get it." The odds of getting this plea overturned on an ineffective assistance of counsel claim are very, very low. But what if you got it? Do you now seriously believe that a trial is the answer, because you're surely not going to get another plea opportunity.
If your lawyer did not give you enough information to make an informed decision on your case and was not prepared if you'd rejected a last moment open plea, then yes, you could appeal on the basis of ineffective assistance of counsel. With a plea, however, even under the best of circumstances, these are almost never granted. Judges allocute defendants on the record with questions that are designed to get around consitutional objections, and if motions to set aside a conviction or withdraw a plea reach a hearing stage, then the defendant's own words at the time of the plea agreement
can be used to impeach him and to cause his motion to be denied.