Okay from what I can see there's a $27 difference in price between the Pro4 and Steel Legend. This is a lot if you're working within a strict budget.
The answer really depends on what is important to you, but I'll try and narrow this down as well as provide some other options.
1. The Steel Legend is not a terrible board, however only buy this if you care about the RGB/colour scheme, noting that these features are what make up a chunk of the $107.
2. If those features don't matter, some solid motherboard options would be:
- MSI B450 Gaming Plus MAX (Well-featured, red theme, slightly better and cheaper than the Steel Legend)
- MSI B450 -A PRO MAX (Virtually the same board as the above, darker theme, slightly cheaper again)
- MSI B450-A PRO (Slightly cheaper again, ^MAX just means BIOS is pre-updated for Ryzen 3000)
3. The Asrock B450 Pro4 isn't a terrible budget choice, however I would fork out the extra dollars for the above boards, unless going cheaper means you can afford the R5 2600. At $71 the below option may also help you do that:
At $60~ and $120~ for the R3 1200 and R5 2600, these are both fantastic CPUs.
From here I would personally either grab the Steel Legend or one of the 3 MSI boards and the R3 1200, and then watch an overclocking tutorial for even better value (not absolutely necessary, but well worth it), or grab the Pro4-F for $71 and muster up some cash for the R5 2600.
This would be a noticeable performance improvement, just forget about overclocking on that particular board.
As a final note, you could also scour the second hand market! Motherboards and processors (especially) are hardy beasts, and you won't come even close to the price/performance options available on the used market. Don't write it off if you're looking to get the most out of your money.
Don't hesitate to come back with any other questions, or even an outline of your situation such as parts you currently have or are looking at, along with your total budget.
Hope this helps!