Ds is a high tech aac user, we use proloquo2go but have experience with others. They all have the same aim but feel very different, ive used proloquo2go, proloquo, grid for iPad and lamp. It very much depends which works best for your little one. They do use different symbol sets, this did affect our decision. We very much like proloquo2go, it is category based, this works for ds. Lamp is better for motor planning, it uses both categories and word associations. You really need some hands on with them. A salt would be able to advise best.
Are you have an assessment or are you self funding? There's pros and cons for both. Is a salt going to help you implement? I only ask as that would significantly affect my decision.
The dedicated devices are very expensive and often unnecessary particularly if you are self funding. As for an iPad if it's being used as an aac it should only be used for that purpose. Ds uses a full size iPad, we have an iPad mini which we use for modelling and ds uses for games. He is not allowed any other apps on his aac (ipad).
Here are your main options, of course there's always others.
Proloquo £87.99 yearly or £8.99 monthly, both options include proloquo coach (this is especially helpful if you don't have salt support).
Proloquo2go £249.99 (50% off in April and October).
Grid for ipad £399.99 (30 day free trial is available).
Lamp words for life £299.99
Touch chat aac £149.99-£299.99
Td snap (tobii dynavox - available with pathways companion app). £9.99 monthly after a 1 month free trial.
There are a number of high tech aac Facebook groups which are great for advice also.