Need advice.

Diana and Weihrauch are the two main high quality spring piston manufacturers and you are unlikely to get a bad gun from either. I have heard people say that Weihrauch is a slight step up from Diana in fit and finish. I have never held a Diana, but I do have a Weihrauch HW30 and the fit and finish is like a $1000+ firearm. I have been known to pick it up, cock and decock it without shooting just because it feels so smooth.

A serious point to take into consideration of spring piston rifles is that the trade-off for high power is size, weight, cocking effort, noise, and forward recoil (jumpiness during firing). As power goes up into the magnum class (ex: Diana 48 at 18-20fpe), enjoyability and achievable accuracy usually go down. This is sometimes worth it if you have specific hunting needs, but that doesn't sound like you. To me, there seems to be a happy point for springers about 12fpe where they have decent power, but still retain pleasant shooting characteristics. Guns in this range won't be overly difficult to cock, won't be too large, and won't jump around too bad during firing (easier to shoot accurately). They still have plenty of power to take game. I made a double mistake when I bought my first springer - low quality and high power. It was not a pleasant experience and I sent it back.

I'd recommend either the Diana 34 or Weihrauch HW95 in .177 (for the flatter trajectory). Of the two, I would totally go with the HW95 due to how impressive the quality and fit and finish is on my HW30. Airguns of Arizona has the HW95 for $315 which I think is still the best deal out there. Another option is a Beeman R9 which is just a rebadged HW95 with nicer stock and muzzle brake (cocking grip) instead of iron sights, but they are a lot more expensive. You could take the irons off of a HW95, add a muzzle brake, and still come out over $100 cheaper.