Two options, two different reasons

One buys speed.
The other lifts limits.

Top-ups
Upgrading your plan
Commitment
None — buy one when you need it
Monthly or yearly, cancel any time
What you are buying
Fast-lane speed, nothing else
Higher caps, premium models, your own branding
Credits
2,500 per €10 — they never expire
Pro 5,000/month; Academy 250 per active student
Cost per 1,000 credits
€4.00
€5.80 inside Pro — the caps come with it
Leaves the relax lane
Untouched and unlimited
Untouched and unlimited
The decision framework

Three questions that point
to the right option.

1️⃣Do credits alone justify an upgrade? — almost never
€10 buys 2,500 fast-lane credits, which is €4.00 per thousand. Pro's included 5,000 credits sit inside a €29 plan, which works out at €5.80 per thousand. If extra speed is genuinely the only thing you need, top-ups are the cheaper route and nothing you buy expires. Upgrade for what else the plan lifts.
2️⃣What actually pushes people to Pro — the student cap
Free covers one educator and ten students on one SprintUp address. Most upgrades happen because there is an eleventh student, because a second site is needed, or because premium models are wanted routinely — not because the credits ran out. Pro lifts those to 50 students and 3 sites with your own branding.
3️⃣When Academy is the only answer — the second educator
Pro is a single-educator plan. The moment a second teacher needs an account of their own, Academy is the option: unlimited educators and students, 5 sites, and 250 fast-lane credits per active student pooled across the school. It starts at €99 a month, which covers your first 25 active students.
A practical guide

Which route for
which situation.

Your situation
Recommended route
Why
Occasional bursts, otherwise inside your plan
Top-ups
Cheaper per credit, and nothing expires
Fast lane most weeks, still one educator, ≤10 students
Top-ups
Pro would not lift anything you are actually hitting
More than 10 students, or premium models routinely
Pro — €29/month
50 students, 3 sites, premium models; credits come along
A second educator, or more than 50 students
Academy — from €99/month
Unlimited educators; 250 credits per active student, pooled