Of course, I feel that there must be more to Tabak's plea than a simple guilty to manslaughter, probably he is trying to claim diminished responsibility due to a psychological condition or a psychotic break due to stress. If that were the case then he could be sent to a hospital and then released as soon as he is better. In some cases that can be a very short amount of time (i.e. a few years). So perhaps that is why they want to present their case to a jury and prove otherwise.
That's very rare, and an extremely difficult outcome to secure so doubt it will go in that direction at all.
Lawyers are usually heavily against going down this route and advise accordingly. Their reasoning being that if given a prison sentence then the person obviously receives a sentence and at the end of that sentence will be released at some point (with a few exceptions of course).
If a hospital order is imposed then the person goes to a (highly) secure (Broadmoor and the likes and by no means a picnic) hospital unit and for an indefinite period of time. After a couple of years or so they could be judged "sane" enough to be released from hospital. However they could also end up in that hospital for the rest of their lives too. The likelihood is that someone deemed unwell enough to merit a hospital order would probably spend longer in a secure hospital than they would had they received a prison sentence instead.
More likely a scenario were Tabak to take this route would be that he'd receive a prison sentence regardless. He might still spend some time in a secure unit if it was deemed that he merited it but on the proviso that once any condition was stabilized that he'd be returned to a prison to continue his sentence.