I have had a very slow progression, and my voice is much more affected than my speech. So, for me, it made sense to try a more costly version of a voice amplifier. Even now, my speech only gets slurred if I'm overly tired, on a good day speech itself is clear.
I was satisfied with how well my voice was amplified. I was not satisfied with the fragility of headset wires. If your speech is slowing, it might be wise to go for an inexpensive voice amplifier, as a temporary measure, and move toward something like a Dynavox.
eBay shows some portable voice amplifiers for less than $40. Right now, there is one used Addvox7 for sale, (seller says in perfect condition) but, I don't think the asking price is all that good considering you'd still have to pay for shipping. It ends up being only like 10% less than what you'd pay new at a medical supply store.
The MDA loaner closet at the ALS clinic you attend should have something for you to borrow. For speech devices they may only have the older Dynavox machines, which are nothing to compare with the newer non-clunky sized models. And, medicare will cover 80% So, on one hand, its an investment, and they'll only pay for one (I think). Hence, its good to figure out what one is best for you.
However, on the other hand. for those who are bulbar onset, waiting until "later on down the road" because you are looking toward when you may need an eye-gaze system probably isn't the way to go, because the time to start using any piece of assistive technology is when you need assistance. rather than waiting until its your only option.
Now would be a good time to focus on researching what features you want, and comparing current models. If you have an iPad, there are text to speak apps available. I personally don't know much about which ones are better than others though.
There have been several threads in the past on this forum discussing the Dynavox and other brands. The newer models will allow you to add custom recordings in your own voice.
So, searching by the word "Dynavox" should bring up a good bit of information. Also, BarryG had many posts on the subject.
As for voice synthesizers, such as ModelTalker, even though this great to do if your voice is still strong, once projection and clarity is compromised, trying to make adequate recordings can be frustrating and exhausting. IMO adding your own recorded words to a Dynavox is going to be more successful, and later, everyone will enjoy being able to hear some words said in your very own voice. Which, of course is another reason to move forward with getting a Dynavox (or another brand of speech generating device).
Here is a helpful link about what Medicare will and will not pay for:
http://www.asha.org/practice/reimbursement/medicare/sgd_policy.htm