I've sailed on both these ships recently. Both the Norwegian Sun and Norwegian Spirit will not disappoint you.
The Spirit has unique Asian decor which is fresh and new on cruise ships sailing from America. But the Sun is three years newer, and has better accommodations in the cabin, except for the bathroom. The Spirit has the largest bathrooms for standard cabins I've ever seen. Glass shower doors beat clinging curtains everytime. The Sun's cabins have refrigerators, although very small in size, it's better than the twice a day refilled ice buckets you get on the Spirit.
The Galaxy of the Stars and the Maharini's Lounge are the Spirit's prime bars-lounges, the Sun's prime nightclubs are Dazzles and the Observation Lounge. For old fashion British pub fun, try out the Henry's Pub on the Spirit, and the Windjamers Bar on the Sun.
As for your reservations questions, below is a listing of the restaurants for both ships. Briefy, you don't need reservations at the main restaurants, but you do at the finer specialty restaurants. At most, only three specialty restaurants require paying a surchage to dine at on either ship.
Norwegian Spirit
The Garden Room (Main Dining Room)
Cuisine: Continental
Dress Code: No shorts or jeans during dinner
Surcharge: No
Reservations Required: No
Windows Restaurant (Main Dining Room)
Cuisine: Continental
Dress Code: No shorts or jeans during dinner
Surcharge: No
Reservations Required: No
Shogun Restaurant
Cuisine: Japanese
Dress Code: No shorts or jeans during dinner
Surcharge: Yes, $12.50 per person
Reservations Required: Yes
Also contains Teppanyaki and Sushi Bar
Le Bistro Restaurant
Cuisine: French
Dress Code: No shorts or jeans during dinner
Surcharge: Yes, $15 per person
Reservations Required: Yes
La Trattoria
Cuisine: Italian
Dress Code: No shorts or jeans during dinner
Surcharge: No
Reservations Required: Yes
Cagney's Steakhouse
Cuisine: Steaks
Dress Code: No shorts or jeans during dinner
Surcharge: Yes, $20 per person
Reservations Required: Yes
Raffles Lido Cafe
Cuisine: Buffet
Dress Code: Casual
Surcharge: No
Reservations Required: No
Blue Lagoon
Cuisine: Food Court, Grill
Dress Code: Casual
Surcharge: No
Reservations Required: No
Beir Garten Grill
Cuisine: German
Dress Code: Casual
Surcharge: No
Reservations Required: No
Norwegian Sun
Four Seasons Main Restaurant (Main Dining Room)
Cuisine: Continental
Dress Code: No shorts or jeans during dinner
Surcharge: No
Reservations Required: No
Seven Seas Main Restaurant (Main Dining Room)
Cuisine: Continental
Dress Code: No shorts or jeans during dinner
Surcharge: No
Reservations Required: No
Pacific Heights
Cuisine: Light/Healthy Cuisine
Dress Code: No shorts or jeans during dinner
Surcharge: No
Reservations Required: Yes
East Meets West (Might be renamed Cagney's Steakhouse upon leaving dry dock)
Cuisine: Steaks
Dress Code: No shorts or jeans during dinner
Surcharge: Yes, $15 per person
Reservations Required: Yes
Ginza Restaurant
Cuisine: Asian (Sushi, Sashimi, Teppanyaki)
Dress Code: No shorts or jeans during dinner
Surcharge: Yes, $12.50 per person
Reservations Required: Yes
Le Bistro French Restaurant
Cuisine: French
Dress Code: No shorts or jeans during dinner
Surcharge: Yes, $15 per person
Reservations Required: Yes
Las Ramblas Tapas Bar & Restaurant
Cuisine: Mexican, Tex-Mex Tapas Bar
Dress Code: No shorts or jeans during dinner
Surcharge: No
Reservations Required: No
Garden Café/Great Outdoor
Cuisine: Informal Outdoor Dining. Breakfast, Lunch, and Snacks
Dress Code: Casual
Surcharge: No
Reservations Required: No
Il Adagio
Cuisine: Italian
Dress Code: No shorts or jeans during dinner
Surcharge: Yes, $12.50 per person
Reservations Required: Yes
I hope this helps