The solicitor I used was, perforce, very much at the inexpensive end, but was experienced in divorce/family law, approachable, and most importantly 100% on my side. He didn't drag things out unnecessarily and kept costs down as much as possible, bless him. He also had a lovely motherly secretary who was free with the tissues as required. These things are more important IMO than a big office and a smart letter heading, but then again, if the right one for your situation is more expensive, I agree with pp that it's an investment worth making. Your STBXH may go for the poshest looking practice to intimidate you, but they're not necessarily going to be any more successful than a lower profile one that knows what they're doing.
I found the solicitors' letters fascinating, watching the cut and thrust between them, the said and unsaid points, the bluff and double bluff. They both knew whether what one party had asked for had a cat's chance in hell when it came before the court. It was like watching fencing: lunge, parry, advance, retreat. "My client wishes..." "My client is prepared/is not prepared to concede..." XH's solicitor cost about twice as much as mine, but was less efficient, and had the disadvantage of a client who made up the facts as he went along. He was at even more of a disadvantage when XH somehow managed to wriggle out of paying him. Mine wanted interim payments, mainly expenses, with the bulk being paid out of the financial settlement at the end.