Planning is very important Barry, I'm at that stage currently, although my plans have been modified many times already. Be prepared for this.

As to tank size, if those are your only choices go for the deeper tank. Your lighting is about 2w per gal level. Should be OK but you may want to experiment with the photo period times.

If you haven't done so already buy a good plant book with the characteristics of the plants you like. Chose slow to medium growers to keep the gardening down and leave out the CO2.

You may want to research the Walstad method as this works for some and requires little intervention when going well.

Substrates are a big issue in my plans ATM the ones I want, Eco complete and Flourite are too expensive here, but generally considered the best option. I think in a Dutch type setup they put garden soil in their gravels as does Ms. Walstad.

Good planning.

Ian