Florida - renting a car in miami - question.

So I'm looking for some advice.

I want to rent a car in Miami for 4days in May this year. Ideally I'd want cheapest possible price and Mustang convertible.

so few questions
1. If I book a convertible on a broker website (Argus Car Hire) - is it more likely or less likely to get a choice between Mustang or Sebring than if I would book directly with Hertz or Avis for example? I.e. If Hertz has choice of two they would give Mustang to clients who booked directly and Sebring to clients who booked via broker.

2. I don't want to end up with Chrystler PT Cruiser Convertible though so if I don't have guarantee that this wouldn't happen, is it better to book standard economy car and seek paid upgrade at the airport upon confirming what is available?

3. are walk in prices much higher than booking online in advance? Or is it possible to get similar deal?

I'd be happy with Mustang either convertible or coupe, Sebring convertible or Crossfire convertible or coupe.... any of those would be perfect but I'm concerned that I may not get it....

I don't think there's a better way to guarantee a Mustang convertible than reserving one from Hertz. Avis and National only let you reserve "Convertible," and National explicitly says it's a Sebring or similar. Hertz lets you specifically pick a Mustang (or, if you're only carrying two people, a Corvette) convertible.
If you reserve something you don't want, you run the risk of getting here and finding that there's no upgrades available. This happens quite often.
I'm not sure; however, most advance bookings let you cancel without penalty. Check with any agency you advance book with, especially if they specify pre-paid (Hertz sometimes lets you do this, and blind booking sites only let you do this).

Thanks for the reply, so let me rephrase the question. What else other than mustang or sebring i could get these days in miami? I can go for "convertible or similar" but wouldnt want to end up with beetle convertible or pt cruiser convertible...

Is it possible to downgrade (and knock off some $$ off the price) as well as upgrade?

Thanks for the reply, so let me rephrase the question. What else other than mustang or sebring i could get these days in miami? I can go for "convertible or similar" but wouldnt want to end up with beetle convertible or pt cruiser convertible...

Is it possible to downgrade (and knock off some $$ off the price) as well as upgrade?

I've never seen a New Beetle or PT Cruiser convertible up for rent in Florida. If you don't pay a premium, you'll get a 2010 or 2011 V6 Mustang convertible, or a Sebring convertible. Hertz has a Corvette convertible, but it costs extra because it's a proper 430hp V8 sports car.

I've never heard about downgrading and getting a lower price. With National, you can usually just take a crummier car, but don't expect a refund of the difference.