Self managed gives you the most freedom of choice. With Agency managed you are limited to NDIS registered providers and will probably incur the NDIS tax (providers charging more to NDIS clients). With plan managed you are limited to what your plan manager is willing to approve at their discretion. With self managed you make your own choices.
If you are not confident you can practice by choosing to self manage a part of your plan and having the rest of plan or agency managed. This can be a good compromise to get confident, especially if you start with self managing something like consumables so you can hunt for a good deal and make your NDIS dollars go further.
If you go with plan managed, make sure you pick a plan manager who understands your goals and shares your values. Otherwise you’ll end up dependent on someone who doesn’t put you and your best interests first.
If you have the confidence, self managed will give you the most freedom and allow you to shop around so you can get the best price. Agree with Sarah, if you go with Agency managed you will not get the best price for any supports or things you need to buy.